Lab 6 - Gravitational and Passive Forces

Transcription

1 Lab 6 Gravitational and Passive Forces L6-1 Name Date Partners L6-1 Lab 6 - Gravitational and Passive Forces Name Date Partners LAB 6 - GRAVITATIONAL AND PASSIVE FORCES And thus Nature will be very conformable to herself and very simple, performing all great Motions of heavenly Bodies And thusby Nature attraction will beof very gravity conformable to herself and very simple, performing all great Motions of heavenly Bodies by Isaac attraction Newton of gravity OBJECTIVES Isaac Newton To explore nature of motion along a vertical line near Earth s surface. OBJECTIVES To extend explanatory power of Newton s laws by inventing an invisible force ( gravitational force) that correctly accounts for falling motion of To explore nature of motion along a vertical line near Earth s surface. objects observed near Earth s surface. To extend examine explanatory magnitude power of Newton s acceleration laws by inventing of a falling an invisible object under force ( influence gravitational of force) gravitational that correctlyforce accounts near for Earth s fallingsurface. motion of objects observed near Earth s surface. To examine motion of an object along an inclined ramp under influence of To examine gravitational magnitude force. of acceleration of a falling object under influence of To gravitational incorporate force frictional near forces Earth sinto surface. Newton s first and second laws of motion. To examine explore interaction motion of forces an object between alongobjects an inclined as described ramp under by Newton s influence third of law of gravitational motion. force. To To incorporate explore tension frictional forces. forces into Newton s first and second laws of motion. To apply Newton s laws of motion to mechanical systems that include tension. To explore interaction forces between objects as described by Newton s third law of motion. OVERVIEW You started To explore your tension study of forces. Newtonian dynamics in Lab 4 by developing concept of force. Initially, when asked to define forces, most people think of a force as an obvious push or To pull, apply such Newton s as a punch laws ofto motion jaw to mechanical or tug systems of a rubber that include band. tension. By studying acceleration that results from a force when little friction is present, we came up with a second OVERVIEW definition of force as that which causes acceleration. These two alternative definitions You started of your force study do not of Newtonian always appear dynamics to be in Lab same. 4 by Pushing developing on a wall concept doesn t of force. seem to Initially, cause when wall asked to move. to define An forces, object most dropped people close think to of a force surface as an of obvious Earth push accelerates or pull, and such yet as re a punch is no tovisible jawpush or or tug pull of aon rubber it. band. By studying acceleration that results from a force when little friction is present, we came up with a second definition of force as that which causes acceleration. These two alternative definitions of force do not always appear to be University University of of Virginia Virginia Physics Physics Department Department PHYS PHYS 1429, 1429, Spring Fall Modified from from P. P. Laws, Laws, D. D. Sokoloff, R. R. Thornton

2 L6-2 Lab 6 Gravitational and Passive Forces L6-2 Lab 6 - Gravitational & Passive Forces same. Pushing on a wall doesn t seem to cause wall to move. An object dropped close to surface of Earth accelerates and yet re is no visible push or pull on it. The genius of Newton was to recognize that he could define net force or combined force The genius of Newton was to recognize that he could define net force or combined force as as that which causes acceleration, and that if obvious applied forces did not account that which causes acceleration, and that if obvious applied forces did not account for for degree of acceleration n re must be or invisible forces present. A prime degree of acceleration n re must be or invisible forces present. A prime example of example of an invisible force is gravitational force attraction of Earth for an invisible force is gravitational force attraction of Earth for objects. objects. When an object falls close to surface of Earth, re is no obvious force being applied When to it. an Whatever object is falls causing close to object surface to move of is invisible. Earth, Most re people is no obvious rar casually force refer being applied to cause to it. of Whatever falling motions is causing as action object of gravity. to move What is invisible. is gravity? Most Can people we describe rar casually gravity as refer justto anor cause force? of Can falling we describe motions its as effects action mamatically? of gravity. CanWhat Newton s is gravity? laws Can be interpreted we describe in suchgravity a way that as y just cananor be used force? mamatical Can we prediction describe of its motions effects mamatically? that are influencedcan by gravity? Newton s laws be interpreted in such a way that y can be used for mamatical prediction of motions that are influenced by gravity? It almost seems that we have to invent an invisible gravitational force to save Newton s second It almost law. seems Since that objects we have near to invent surface an invisible of gravitational Earth fall force with to a constant save Newton s acceleration, second we law. will Since use objects Newton s nearsecond surface law of to show Earththat fallre with amust constant be a acceleration, constant (gravitational) we will use force Newton s acting second law object. to show that re must be a constant (gravitational) force acting on object. Finding invisible forces (forces without an obvious agent to produce m) is often hard because Finding some invisible of forces m (forces are not without active anforces. obvious agent Rar, to produce y are m) passive is often forces, hard because such as normal some offorces, m are which not active crop forces. up only Rar, in response y are passive to active forces, ones. such as (In normal case forces, of which normal forces, crop up only active in response forces are to active ones ones. like (In push case you ofexert normal on forces, a wall or active gravitational forces are ones pull on like a book push sitting you on exert a table.) on a wall or gravitational pull on a book sitting on a table.) Frictional and tension forces are or examples of passive forces. The passive nature of friction Frictional and tension forces are or examples of passive forces. The passive nature of is obvious when you think of an object like a block being pulled along a rough surface. There is friction is obvious when you think of an object like a block being pulled along a rough an applied force (active) in one direction and a frictional force in or direction that opposes surface. There is an applied force (active) in one direction and a frictional force in motion. If applied force is discontinued, block will slow down to rest but it will not or direction that opposes motion. If applied force is discontinued, block start moving in opposite direction due to friction. This is because frictional force is will passive slow and down stops to acting rest but as soon it will as not block start moving comes to in rest. opposite direction due to friction. This is because frictional force is passive and stops acting as soon as block comes to Likewise, rest. tension forces, such as those exerted by a rope pulling on an object can exist only when re is an active force pulling on or end of rope. Likewise, tension forces, such as those exerted by a rope pulling on an object can exist only In this when lab you re will is first an active study vertical force pulling motion on and or gravitational end of force. rope. Then you will examine motion of an object along an inclined ramp. You will use Newton s laws of motion as a In working this lab hyposis you will to first invent study frictional vertical motion and tension and forces. gravitational Along way force. youthen will examine you will examine Newton s third motion law of of motion. an object along an inclined ramp. You will use Newton s laws of motion as a working hyposis to invent frictional and tension forces. Along way you INVESTIGATION will examine Newton s 1: third Motion law of and motion. Gravity PHYS University 1429, of Spring Virginia 211 Physics Department

3 Lab 6 Gravitational Lab 6 and - Gravitational Passive & Forces Passive Forces L6-3 L6-3 INVESTIGATION 1: MOTION AND GRAVITY Let s begin study of phenomenon of gravity by examining motion of an object such as Let s a ball begin when study it is allowed of phenomenon to fall vertically of gravity nearby examining surface of Earth. This study motion isof not an easy, object because such as a ball motion when happens it is allowed veryto quickly! fall vertically You near surface of Earth. This study is not easy, because motion can first predict what kind of motion ball undergoes by tossing a ball in happens very quickly! You can first predict what kind of motion ball laboratory several undergoes times and by seeing tossing what a ball you in think laboratory is goingseveral on. times and seeing A falling motion what is too you fast think to is observe going on. carefully by eye. You will need aid of motion detector A falling and motion computer is too to fast examine to observe carefully motionby quantitatively. eye. You will Fortunately, motion aid detector of can motion do measurements detector and computer just fastto enough examine to graph motion this need quantitatively. Fortunately, motion detector can do measurements motion. To carry just out fast your enough measurements to graph this you motion. will need To carry out your measurements you will need motion detector motion detector basketball table clamps, rods, etc. 3 m tape measure softball 4 x 4 wood block table clamps, rods, etc. Activity 1-1: Motion of a Falling Ball 3 m tape measure Prediction 1-1: Suppose that you drop ball from height of about 2 m above floor, releasing it from rest. On axes that follow, sketch your predictions for velocity 4 x 4 wood block time and acceleration time graphs of ball s motion. Assume that positive y direction is upward. Activity 1-1: Motion of a Falling Ball PHYS 1429, Spring 211 PREDICTION Prediction 1-1: Suppose that you drop ball from height of about 2 m above floor, releasing it from rest. On axes that follow, sketch your predictions for velocity time and acceleration time graphs of ball s motion. Assume that positive y direction is upward. - Acceleration Velocity 2 Acceleration Velocity Time (s) Test your predictions. 1 You have 2 previously 3 used motion 4 detector to 5 examine motion of your body and of a cart. The motion of a falling ball takes place much faster PREDICTION 1 2 Time (s) Test your predictions. You have previously used motion detector to examine motion of your body and of a cart. The motion of a falling ball takes place much faster. While you can use motion detector to measure position, velocity, and acceleration, you will have to collect data at a faster rate than you have before.

4 L6-4 Lab 6 Gravitational and Passive Forces The easiest way to examine motion of falling ball is to mount motion detector as high up as you can and to use a large ball that is not too light (like a basketball rar than a beach ball). It is essential to keep your hands and rest of your body out of way of motion detector after ball is released. This will be difficult and may take a number of tries. It also will take some care to identify which portions of your graphs correspond to actual downward motion of ball and which portions are irrelevant. 1. Use table clamps, rods, and right angle devices to attach motion detector as high above floor above floor as length of rod allows, with detector looking straight downward, as shown on right. 2. Set motion detector on wide beam setting. 3. Open experiment file called L6.1-1 Falling Ball. The axes will look similar to ones on which you made your predictions. This experiment file will also set data collection to a faster rate than has been used before (1 points/s). Because motion detector is pointing downward - in negative y direction software has been set up to make distance away from detector negative. 4. Hold ball at least 2 cm directly below motion detector and at least 18 cm above floor. Remember that your hands and body must be completely out of path of falling ball, so detector will see ball and not your hands or body whole way down. 5. When everything is ready, begin graphing and release ball as soon as you hear clicks from motion detector. 6. Adjust axes if necessary to display velocity and acceleration as clearly as possible. Do not erase data so that graphs can be later used for comparison. 7. Use mouse to highlight time interval during which ball was falling freely. Question 1-1: What does nature of motion look like constant velocity, constant acceleration, increasing acceleration, decreasing acceleration, or or (describe, if or)? Discuss how your observations compare with your predictions.

5 Lab 6 Gravitational and Passive Forces L6-5 Question 1-2: Is acceleration of ball positive or negative as it falls? Does this sign agree with way that velocity appears to be changing on velocity-time graph? Discuss. Activity 1-2: The Magnitude of Gravitational Acceleration As you saw in Lab 3, you can find a value for average acceleration in various ways. One method is to read average value from acceleration - time graph. Anor is to find slope of velocity time graph. 1. Use statistics features of software to read average value of acceleration during time interval from just after ball began falling (beginning of uniform acceleration) to just before ball stopped falling (end of uniform acceleration). Average acceleration: ± m/s 2 2. Use fit routine to find mamatical relationship between velocity and time during same time interval as in step 1. Write below equation you find from fit that relates velocity (v) to time (t). 3. From equation in step 2, what is value of acceleration? Average acceleration: ± m/s 2 Question 1-3: Discuss how well two values for gravitational acceleration agree with each or. Activity 1-3: Motion Up and Down Prediction 1-2: Suppose that you toss a ball upward and analyze motion as it moves up, reaches its highest point, and falls back down. Is acceleration of ball same or different during three parts of motion moving upward, momentarily at highest point, and moving downward? Explain.

6 highest point, and moving downward? Explain. Moving L6-6 upward: Lab 6 Gravitational and Passive Forces Moving upward: Momentarily at highest point: Momentarily at highest point: Moving Moving downward: downward: Prediction 1-3: Sketch on axes below your predictions of velocity - time and Prediction 1-3: Sketch on axes below your predictions of velocity - time and acceleration time graphs for entire motion of ball from moment it leaves your acceleration - time graphs for entire motion of ball from moment it leaves your hand going up until just before it returns to your hand. Assume that positive direction hand going up until just before it returns to your hand. Assume that positive direction is upward. is upward. 2 + PREDICTION Time when ball reaches highest point Time 4 5 Throwing ball upward and keeping it in range of motion detector is harder to do Throwing than dropping ball upward ball. and Try keeping to throw it in range ball up of directly motion under detector is motion harder detector. to do than It dropping ball. Try to throw ball up directly under motion detector. It may take a may take a number of tries. Again be sure that your body is not seen by motion number of tries. Again be sure that your body is not seen by motion detector. detector. 1. The same experiment file L6.1-1 Falling Ball used in Activity 1-1 should work in this University activity of Virginia as well. Physics Keep Department graphs from Activity Modified 1-1 on from P. screen Laws, D.. The Sokoloff, new R. data Thornton will PHYS 1429, Spring 211 be in a different color. You can also click on Data icon and choose not to display a particular data run. 2. When everything is ready, begin graphing, and when you hear clicking begin, toss ball up toward motion detector. Toss ball as high as you can, but remember that it should never get closer than 2 cm below motion detector. A short quick throw will work best. Repeat until you get a throw where you are sure ball went straight up and down directly below detector. You can choose under Experiment to delete last data run if you do

7 Lab 6 Gravitational and Passive Forces L6-7 not want to keep it. You can also click on a graph and use DELETE key to remove it. You may need to make several tries before you decide to keep data. Let all group members try, if you have time. 3. When you obtain a good run, print one set of graphs for your group and include m in your report. Your print out should include your best data from Activity 1-1 AND Activity Label portions of printed graphs that show ball s up and down motion. Also label with an arrow instant in time when ball reached its highest point. Question 1-4: Qualitatively compare acceleration during three parts of motion on way up, at highest point, and on way down. Discuss your observations based on sign of change in velocity. How well do you observations agree with your predictions? On way up: At highest point: On way down: Question 1-5: There is a common idea that acceleration of ball must be zero when it stops. If this were true, could velocity of ball change L6-8 once it stopped? Discuss. Activity 1-4: Vertical Motion with Air Resistance In this Activity you will use motion detector to examine motion of a paper coffee filter falling from rest. In addition to setup in Activity 1-1 you will need a flat-bottomed paper coffee filter - type with folds along sides 1. Use experiment file L6.1-4 Falling Filter. As usual, wait until you hear motion detector click Activity 1-4 (Optional): Vertical Motio In this Activity you will use motion filter falling from rest. In addition to a flat-bottomed paper coffee filter - th 1. Use ex wait until release shown on record velocity a graphs mo a good gra side out of 2. Be sure to keep your body out of

8 L6-8 Lab 6 Gravitational and Passive Forces and n release coffee filter with flat bottom facing down as shown on left. If necessary, increase time range to record complete motion of filter and adjust velocity and acceleration axes if necessary to display graphs more clearly. It may take you several tries to obtain a good graph, because coffee filter tends to float to side out of range of motion detector. 2. Be sure to keep your body out of way of motion detector. 3. Print out one set of graphs and include m with your report. Question 1-6: Compare graphs to those for falling ball. Does filter also appear to fall with a constant acceleration? If not, how would you describe motion? Is velocity changing as filter falls? If so, how? Question 1-7: Based on Newton s laws of motion, do you think that gravitational force is only force acting on filter? If re is anor force, what is its direction and how does its magnitude compare to gravitational force? Discuss. Activity 1-5: Motion Along an Inclined Ramp Anor common motion involving gravitational force is that of an object along an inclined ramp. Prediction 1-4: Consider a very low-friction cart moving along an inclined ramp, as shown above. The cart is given a push up ramp and released, and its motion is graphed using motion detector at top of ramp. [The direction toward motion detector is positive.] Sketch on axes below your predictions of velocity - time and acceleration - time graphs for motion of cart.

9 Lab 6 Gravitational and Passive Forces L PREDICTION Acceleration Velocity Time (s) Prediction 1-5: How do you predict magnitude of acceleration of cart will compare to acceleration you determined in Activity 1-2 for a ball falling straight downward? Explain. To test your predictions, you will need in addition to equipment used above motion cart 2-m motion track and support to raise one end about 1 cm 1. Set up ramp with motion detector at high end. Use a wooden block to elevate one end of ramp by 1 cm or so. Be sure that motion detector sees cart over whole length of ramp. 2. Open experiment file called L6.1-5 Inclined Ramp to display velocity time and acceleration time plots that were shown previously for Prediction 1-4. As before, software has been set up to consider motion toward detector positive so that positive direction of motion is up ramp. 3. Hold cart at bottom of ramp and begin graphing. When you hear clicks of motion detector, give cart a push up ramp and release it. DO NOT PUSH THE CART HARD ENOUGH FOR IT TO HIT THE MOTION DETECTOR OR FALL OFF THE TRACK!! PRACTICE A COUPLE OF TIMES! The graph should include all three parts of motion up ramp, at its highest point, and on its way down and cart should never get closer than 2 cm from motion detector. Repeat until you have good graphs. Only keep your good data. 4. Print out one set of graphs for your group and include m with your report.

10 L6-1 Lab 6 Gravitational and Passive Forces 5. Use an arrow to mark instant when cart was at its highest point along track on both velocity and acceleration graph. 6. Use statistics or linear fit features of software to measure average acceleration of cart during time interval after it was released and before it was stopped at bottom of ramp. Average acceleration: ± m/s 2 Question 1-8: Did your graphs agree with your predictions? Does this motion appear to be with a constant acceleration? Question 1-9: How did magnitude of acceleration compare with that for ball falling which you determined in Activity 1-2? Is this what you predicted? Was this motion caused by gravitational force? Why isn t acceleration same as for ball falling? Question 1-1: What fraction of falling-ball acceleration did cart experience? can you explain why this fracion is less than one? % INVESTIGATION 2: Newton s Laws when Friction is Present In previous labs we have worked hard to create situations where we could ignore effects of friction. We have concentrated on applied forces involving pushes and pulls that we can see and measure directly. The time has come to take friction into account. You can make observations by applying a force directly to your force probe mounted on a block (with significant friction) and comparing block s acceleration to acceleration of cart (with negligible friction). To make observations on effects of friction you will need force probe motion detector

11 that we can see and measure directly. The time has come to take friction into account. You can make observations by applying a force directly to your force probe mounted on a Lab 6 Gravitational and Passive Forces L6-11 block (with significant friction) and comparing block s acceleration to acceleration of cart (with negligible friction). string ( 3 cm. long with loops on each end). To make observations on effects of friction you will need! force 2-mprobe motion track! 2-m motion track! motion wooddetector friction block! wood friction block! string ( ~3 cm. long with loops on each end). Activity Activity 2-1: 2-1: Static Static and and Kinetic Kinetic Frictional Frictional Forces Forces If frictional force is equal in magnitude to applied force, n according to If frictional force is equal in magnitude to applied force, n according to Newton s first Newton s first law, block must eir remain at rest or move with a constant velocity. law, block must eir remain at rest or move with a constant velocity. The frictional forces when two objects are sliding along each or are called kinetic The frictional forces when two objects are sliding along each or are called kinetic frictional frictional forces. If objects are not sliding along each or, n frictional forces forces. If objects are not sliding along each or, n frictional forces are called static are frictional called static forces. frictional In this Activity forces. you In will this examine Activity wher you will frictional examine force wher is different frictional when force an object is different is at rest when (statican friction) object than is at when rest it (static is sliding friction) along athan track when (kinetic it is friction). sliding along a track (kinetic friction). Prediction 2-1: A block is sitting on a table, as shown above. You pull on a string attached Prediction to 2-1: block, A block and is sitting block ondoesn t a table, move as shown because above. You frictional pull onforce a string opposes attached to block, and block doesn t move because frictional force opposes your University pull. You of pull Virginia harder, Physics anddepartment eventually block begins Modified to slide from along P. Laws, D. table Sokoloff, top. How R. Thornton PHYS does1429, frictional Spring 211 force just before block started sliding (meaning that pull force did not change in that instance) compare to frictional force when block is actually sliding? Explain. You can test your prediction by mounting force probe on a wooden block and pulling block along table top (not on aluminum track). 1. Measure mass of block and force probe: Mass of block: kg. Mass of probe: kg 2. Set up block, string, force probe and motion detector as shown below. Make sure that wood side of block is down. Set force probe on top of friction block. Place all four 1/2 kg masses on force probe to make overall mass about 2.5 kg.

12 L6-12 Lab 6 Gravitational and Passive Forces 3. Prepare to graph velocity and force by opening experiment file called L6.2-1 Static and Kinetic Friction. 4. Zero force probe with nothing pulling on it. Begin graphing with string loose, n gradually pull very gently on force probe with string and increase force Lab 6 - Gravitational very slowly. & Be Passive sureforces that you pull horizontally not at an angle up or down. When L6-13 block begins to move, pull only hard enough to keep it moving with a small velocity, which is as constant (steady) as possible. Question 2-1: Does block feel a force on it due to string, and yet not move? 5. Do not erase your data so that you can use m for comparison with those in Why? What happens to frictional force just as block begins to slide? Does this Activity 2-2. Do not yet print out data. agree with your prediction? Explain. Question 2-1: Does block feel a force on it due to string, and yet not move? Why? What happens to frictional force just as block begins to slide? Does this agree with your prediction? Explain. Question 2-2: Note time on your force graph when block just began to slide (you ll mark it later after you ve printed graphs). From graph, how do you know when Question this time 2-2: is? Note time on your force graph when block just began to slide (you ll mark it later after you ve printed graphs). From graph, how do you know when this time is? Activity 2-2: Frictional Force and Normal Force Activity 2-2: Frictional Force and Normal Force The frictional force is nearly same regardless of how fast object moves, but re is usually a difference between static and kinetic frictional forces. Now you will examine The frictional force is nearly same regardless of how fast object moves, but re is usually a difference between static and kinetic frictional forces. Now you will examine if frictional if frictional forces depend on force between surface and object normal force. forces depend on force between surface and object normal force. All All of of forces forces exerted exerted on on a block a block being being pulled pulled on a on table a table top are top shown are shown in diagram in diagram below. below. Surfaces Surfaces like a wall like or a table wall top or a can table exert top a force can perpendicular exert a force to perpendicular surface called to a normal surface called force. a Innormal caseforce. of block In on case table, of block table exerts on a normal table, force table upward exerts on a block, normal force and upward Earth exerts on a block, gravitational and force Earth ( exerts weight a ofgravitational block) downward force ( on weight block. of block) downward on block. Since we know that block doesn t move up or down as it sits at rest or slides along Since table s we know surface, that block combined doesn t (net) movevertical up or down force asmust it sits be at rest zero oraccording slides alongto Newton s table s first surface, law. So combined magnitude (net) vertical of force normal must be force zeromust according just to equal Newton s magnitude first law. Soof gravitational force. Using symbols in diagram, we obtain FG = FN = mg where m is total mass of object, and g is gravitational acceleration, 9.8 m/s 2.

13 Lab 6 Gravitational and Passive Forces L6-13 magnitude of normal force must just equal magnitude of gravitational force. Using symbols in diagram, we obtain F G = F N = mg where m is total mass of object, and g is gravitational acceleration, 9.8 m/s 2. Prediction 2-2: If you change normal force exerted by table on block (by adding or removing masses from force probe), what will happen to frictional force? Explain. Test your prediction. Use same setup and experiment file, L6.2-1 Static and Kinetic Friction, as in Activity Remove all but two of 1/2 kg masses from top of force probe. [The new total mass should be about 1.5 kg.] 2. Zero force probe with nothing pulling on it. The data from Activity 2-1 is still being displayed on computer screen. As in Activity 2-1, begin graphing with string loose, n gradually pull very gently on force probe with string, and increase force very slowly. Be sure that you pull horizontally not at an angle up or down. When block begins to move, pull only hard enough to keep it moving with a small velocity that is as constant (steady) as possible. 3. Both sets of data with different masses should be displayed. Question 2-3: Compare two sets of graphs. In what ways are y similar, and in what ways are y different? Are kinetic frictional forces same or different? Discuss. 4. For each of runs, use statistics or linear fit features of software to measure average frictional forces during time intervals when block was moving with a constant velocity. Record systems masses and frictional forces in appropriate row of Table 6.1. [You can choose which run you are displaying by clicking on and off Run # under Data icon.] 5. Calculate normal forces and coefficient of friction ( ratio of frictional force to normal force: µ = F f / FN ) for each run and record in Table Print out one graph containing all se data for your group. Carefully mark on your graph total mass for each set of data. 7. Use an arrow to mark time on your force graph when block just began to slide (see Question 2-2).

14 L6-14 Lab 6 Gravitational and Passive Forces System: block, force probe and mass 2.5 kg masses 4.5 kg masses Total Mass (kg) Normal force (N) Average kinetic frictional force (N) Coefficient of friction Table 6.1: Question 2-4: Discuss assertion that µ is approximately constant. NOTE: Pulling block with constant force is very difficult. In our experience we observe variance of measured force for same conditions to be as much as 5%. Keep this value in mind while discussing your final results. INVESTIGATION 3: Newton s Third Law All individual forces on an object can be traced to an interaction between it and anor object. For example, we believe that while falling ball is experiencing a gravitational force exerted by Earth on it, ball is exerting a force back on Earth. In this investigation we want to compare forces exerted by interacting objects on each or. What factors might determine forces between objects? Is re a general law that relates se forces? We will begin our study of interaction forces by examining forces each person exerts on or in a tug-of-war. Let s start with a couple of predictions. Prediction 3-1: Suppose that you have a tug-of-war with someone who is same size L6-16 and weight as you. You both pull as hard as you can, and it is alab stand-off. 6 - Gravitational One of you & Passive might Forces move a little in one direction or or, but mostly you are both at rest. Predict relative magnitudes of forces between person 1 and person 2. Place a Predict relative magnitudes of forces between person 1 and person 2. Place a check next check next to your prediction. to your prediction. Person 1 exerts a larger force on person 2. Person 1 exerts a larger force on person 2. The people exert same size force on each or. Person 2 exerts Thea people larger exert force on same person size1. force on each or. Prediction University of3-2: Virginia Suppose Physics Department now that you have Modified a tug-of-war from P. with Laws, someone D. Sokoloff, who R. Thornton is much smaller PHYS 1429, and Fall lighter 215than you. As before, you both pull as hard as you can, and it is a stand-off. One of you might move a little in one direction or or, but mostly you are both at rest.

15 check next to your prediction. Person Predict Lab 6 Gravitational relative 1 exerts and magnitudes a Passive larger Forces force of on person forces 2. between person 1 and person 2. Place L6-15 a The check next to people your exert prediction. same size force on each or. Person 21 exerts Person a larger 2 exerts force a larger on person force on person 1. Prediction The 3-2: people 3-2: Suppose exert now same that size you force have on a a each tug-of-war or. with with someone someone who who is much is much smaller you. As before, you both pull as hard you can, and it is a stand- Person and lighter 2 exerts than a larger you. As force before, on person you 1. both pull as hard as you can, it is a stand-off. One ofone youof might you move might a little move ina one little direction in one or direction or, or but mostly or, you but are mostly both at you are both Prediction rest. at rest. 3-2: Suppose now that you have a tug-of-war with someone who is much smaller and lighter than you. As before, you both pull as hard as you can, and it is a stand-off. One of you might move a little in one direction or or, but mostly you are both at rest. Predict relative magnitudes of forces between person 1 and person 2. Place a check Predict next to relative your prediction. magnitudes of forces between person 1 and person 2. Place a check next to your prediction. Person Predict relative 1 exerts magnitudes a larger force of on person forces 2. between person 1 and person 2. Place a The check next to people your exert prediction. Person 1same exertssize a larger force force on on each person or. 2. Person 21 exerts The a people larger exert force on same person size1. 2. force on each or. Prediction The 3-3: people Again, exert suppose same that size you force have on each a tug-of-war or. with someone who is much smaller and lighter than Person you. 2 exerts This a larger time force lighter on person person 1. Person 2 exerts a larger force on person 1. is on a skateboard, and with some Prediction effort you 3-3: are Again, able to suppose pull him that or you her have along a tug-of-war floor. with someone who is much Prediction 3-3: Again, suppose that you have a tug-of-war with someone who is much smaller and lighter than you. This time lighter person is on a skateboard, and with some smaller and lighter than you. This time lighter person is on a skateboard, and with effort you are able to pull him or her along floor. some effort you are able to pull him or her along floor. PHYS 1429, Spring 211 Predict relative magnitudes of forces between person 1 and person 2. Place a check next to your prediction. PHYS 1429, Spring 211 Person 1 exerts a larger force on person 2. The people exert same size force on each or. Person 2 exerts a larger force on person 1.

16 L6-16 Lab 6 Gravitational and Passive Forces INVESTIGATION 4: TENSION FORCES INVESTIGATION 4: Tension Forces When you pullon a rope attached to a crate, your pull is somehow transmitted down down rope torope crate. to Tension crate. is Tension nameis given to name forces given transmitted to forces along transmitted stretched strings, along stretched ropes, rubber strings, bands, ropes, springs, rubber and wires. bands, springs, and wires. Is whole force you apply transmitted to crate, or is Is pull at whole or force end you larger apply or smaller? transmitted Does to it matter crate, how or is long pull rope at is? How or isend force larger magically or smaller? transmitted Does matter along how rope? long These are rope some is? of How questions is youforce will examine in this investigation. University Obviously, of Virginia ropephysics by itself Department is unable to exert a force Modified on from crate P. Laws, if youd. are Sokoloff, not pulling R. Thornton on PHYS or1429, end. Spring Thus, 211 tension forces are passive just like frictional and normal forces. They act only in response to an active force like your pull. Prediction 4-1: If you apply a force to end of a rope as in picture above, is whole force transmitted to crate, or is force at crate smaller or larger than your pull? Prediction 4-2: What happens when a string is hung around a pulley? Is tension force still transmitted fully from one end of string to or? To test your prediction, in addition to equipment listed above you will need low-friction pulley 2-m motion track level low friction cart long piece of string Activity 4-1: Tension When a String Changes Direction 1. Attach force probe 1 securely to cart, and set up cart, track, pulley, string, and force probes.

17 Lab 6 Gravitational and Passive Forces L Open experiment file L6.4-1 Tension Forces. You will have graphs of two force probes versus time. 3. Zero both force probes with string loose. Hold cart to keep it from moving. You can use track s end clamp to secure cart from moving. Then begin graphing while pulling on force probe 2. Pull softly at first, n harder, and n alternately soft and hard. Do not exceed 1 N. 4. Print out one set of graphs for your group report. Question 4-1: Based on your observations of readings of two force probes, was pull you exerted on force probe 2 transmitted undiminished to force probe 1 even though it went through a right angle bend? How does this compare to your prediction? Discuss. Activity 4-2: Tension and Newton s Second Law Prediction 4-3: Suppose that in place of force probe 2 you hang a 1 g mass from end of string. With cart held at rest, what value do you expect force probe 1 to read? Be quantitative. Explain.

18 L6-18 Lab 6 Gravitational and Passive Forces Prediction 4-4: Suppose that you hang same 1 g mass but release cart and let it accelerate as mass falls. Do you expect force probe 1 to read same force or a larger or smaller force as cart accelerates? Explain. To test your predictions you will need following in addition to equipment for Activity 4-1: 1 g mass 1. Use same setup as for Activity 4-1, but replace force probe 2 with a hanging mass and set up motion detector. 2. Determine mass of cart and force probe: g 3. Open experiment file called L6.4-3 Tension and N2. 4. Be sure that motion detector can see cart all way to end of track. 5. Hang 1 g mass from end of string, and hold cart at rest at least 2 cm away from motion detector. 6. Just before you are ready to start zero force probe with string loose. Let 1 g mass hang without swinging and n begin graphing. Hold cart for first second after you hear clicks of motion detector and n release it. Be sure to stop cart before it comes to end of track. 7. Print out one set of graphs for your group. Use arrows on your force and acceleration graphs to indicate time when cart was released. 8. Use statistics or linear fit features of software to measure average value of tension measured by force probe before cart was released and while it was accelerating. Also measure average value of acceleration during time interval while cart had a fairly constant acceleration. Average tension with cart at rest: ± N Average tension with cart accelerating: ± N Average acceleration: ± m/s 2 Question 4-2: How did tension while cart was at rest compare to weight of hanging mass? [Hint: Remember that weight mg.] Did this agree with your prediction? Was force applied to end of string by hanging mass transmitted undiminished to cart, as you observed in previous activity?

19 Lab 6 Gravitational and Passive Forces L6-19 Question 4-3: Did anything happen to tension after cart was released? How did value of tension as cart was accelerating compare to weight of hanging mass? Did this agree with your prediction? Discuss. Comment: When cart is released, both cart and hanging mass must always move with same velocity since y are connected by string as a single system. Thus, cart and mass must have same acceleration. According to Newton s second law, combined (net) force on cart must equal its mass times acceleration, and combined (net) force on hanging mass must equal its mass times same acceleration. Question 4-4: Use Newton s second law to explain your answer to Question 4-3. Why must tension in string be smaller than weight of hanging mass if hanging mass is to accelerate downward? Please clean up your lab area