Chapter 4. Forces and the Laws of Motion. CH 4 Forces and the Laws of Motion.notebook. April 09, Changes in Motion. A. Force

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1 CH 4 Forces and the Laws of Motion.notebook Chapter 4 A. Force April 09, 2015 Changes in Motion Forces and the Laws of Motion 1. Defined as the cause of an acceleration, or the change in an object s motion, or simply a push or pull exerted on some object Forces cause change in motion a. stopping a moving object Unit of force is the newton (N) a. newton is defined as the amount of force that, when acting on a 1 kg mass, produces an acceleration of 1 m/s 2 (1 N = 1 kg 1 m/s 2 ) b. starting an object from rest c. changing direction of an object b. weight is actually a unit of force---why?? 1 lb = N = kg; 1 N = lb = kg Forces can act through contact or at a distance a. contact forces-forces that arise from the physical contact of two objects B. Force Diagrams 1. The effects of forces depend on their magnitude and direction, so force is a vector quantity b. field forces-forces that can exist between objects, even in the absence of physical contact between the objects 1

2 CH 4 Forces and the Laws of Motion.notebook 2. Free body diagrams (see ex., pg. 123) a. vectors are used to show all the forces important in a situation Newton s First Law---Inertia A. Stated---An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless it experiences a net external force April 09, 2015 b. forces are drawn as if they act on a particular point c. good idea to use simple shapes to illustrate different objects in the diagram 1. Inertia is defined as the tendency of an object not to accelerate 2. Newton s first law is often referred to as the law of inertia since his law says that when the net external force on an object is zero, its acceleration is zero a. external force-a single force that acts on an object as a result of the interaction between the object and its environment 3. An object s inertia is directly proportional to its mass; the greater the mass of an object, the less it accelerates when a force is applied to it b. net external force-the vector sum of all the forces acting on a body (the resultant of multiple force vectors) 2

3 CH 4 Forces and the Laws of Motion.notebook April 09, 2015 B. Equilibrium is a term associated with Newton s first law which describes things that are either at rest, or in motion with a constant velocity. The net external force acting on an object must be zero for it to be in equilibrium. Newton s Second Law---F = m a 1. Stated---The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the mass of the object 2. Equationally, net external force = mass x acceleration, F = m a Newton s Third Law 3. Often, in problem solving, forces are broken into x and y components 1. Stated---If two bodies interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction 2. This law is more simply stated for every action there is an equal and opposite reaction 3. Out of this law, comes the idea of an action-reaction pair of forces a. action-reaction pair-a pair of simultaneous equal but opposite forces resulting from the interaction of two objects b. action-reaction pairs don t usually result in equilibrium; movement of an object usually results this can happen since the pair is acting on different objects 3

4 CH 4 Forces and the Laws of Motion.notebook 4. Field forces also occur in pairs, so, according to Newton s law, as an object accelerates toward earth, earth also accelerates toward the object IV. Everyday Forces A. Weight April 09, Defined as the magnitude of the force of gravity acting on an object 2. Weight can be determined from F=ma, where g is used for a 3. Weight varies with location, since the acceleration due to gravity varies with altitude, depends on the planet you re on B. The Normal Force 1. Defined as a contact force exerted by one object on another in a direction perpendicular to the surface of contact 2. The normal force in always perpendicular to the surface, not necessarily opposite the force of gravity 1. Types of Friction C. The Force of Friction 3. The normal force is usually used in problem-solving and becomes very important when determining forces for objects along an incline a. static friction-the force exerted on a motionless body by its environment to resist an external force 4

5 CH 4 Forces and the Laws of Motion.notebook b. kinetic friction-the force exerted on a moving object (kinetic is less than static c. air resistance is also a force of friction, sometimes referred to as fluid friction the friction caused by air resistance is proportional to the speed of the object in motion April 09, 2015 a. these ratios are called coefficients of friction, and can be calculated for static and kinetic friction situations 2. The amount of the force of friction depends on the surfaces in contact; different surfaces in contact have different ratios of friction force compared to the normal force b. coefficient of friction (m) = force of friction (ff)/normal force (fn) 5

6 Build a Water Tower Materials: Objective: Newspaper Masking tape Golf Balls for testing purposes only You will have 15 minutes to design a Water Tower using only the materials provided. The Water Tower should be able to hold a large amount of golf balls without collapsing or losing golf balls. You will have 25 minutes to construct your Water Tower. Qualifying Requirements: Scoring: - Lowest golf ball must be at least 10 inches off the ground - Water tower must hold a minimum of 5 golf balls - Water towers that do not meet these requirements will be disqualified At the end of the 25 minute construction period, ONE team member will take the water tower and stand in the designated sport. After the team s water tower has been tested for the qualifying requirements and passes, the team member will advance to the golf ball station. At the golf ball station, the team member will put golf balls in the water tower 1 at a time until the water tower collapses or the golf balls fall out of the water tower. When a golf ball falls off the tower or the tower collapses, the testing is over. The point totals will be determined by multiplying the # of golf balls the tower can hold (without a golf ball falling off or the tower falling over) by the height (in inches) of the water tower. In case of a tie, the tower that weighs the least will be the winner. Questions that correspond with the activity will be on a separate sheet.

7 Newton s Laws of Motion Video Objective: Create a video that captures, demonstrates, and explains Newton s 3 Laws of Motion. Video needs to be 2 minutes to 4 minutes in length. Multiple demonstrations are encouraged to be used. Videos will be filmed using the camera on a phone and will be edited using WeVideo in Google. Ideas for demonstrations:

8 Physics Egg Drop Challenge Group Members Objective: The purpose of the lesson is to understand the forces acting on a falling object. The lesson will focus on the acceleration due to the force of gravity, air resistance, impulse, and Newton s Laws of motion. The students will then design their egg drop vehicles and build them. Group Size: 2 Students Things to keep in mind: - The egg cannot be completely enclosed as that would restrict breathing as well as spoil the free faller s view. - No parachutes or wings can be used to inhibit the free fall experience, therefore the object should fall directly downward. - Object must fit into an area of 6 x6 x12. Materials List: Each group may select only four (4) of the materials from the following list. Each material is limited in quantity to encourage careful planning of the egg drop vehicle. 1. Cotton balls (15) 2. Newspaper (2 sheets) 3. Printer paper (5 sheets) 4. Egg carton (6 cups, connected) 5. Sandwich bags (2) 6. Bendy straws (10) 7. Masking tape (3 meters) 8. Popsicle sticks (10) 9. Toilet paper (2 meters) 10. Rubber bands (10) 11. Marshmallows (10) Find the Masses: Egg drop container (minus egg): Egg: Total Mass (container with egg): Calculate the Force (Newton s Second Law) using the F=ma with proper units: m = Kg F=ma F = ( Kg) (9.81 m/s 2 )= N

9 Time for drop: Calculate the average velocity for the drop. Did your egg survive the fall? Explain why or why not? List three things that you could have done differently to make sure that your egg survives the fall? Explain how your device uses opposite forces (Newton s Third Law) and minimizes inertia (Newton s First and Second Law) to give the best free fall experience safely.

10 Static and Kinetic Friction Computer 12 If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving. Static friction is the force that is counters your force on the box. If you apply a light horizontal push that does not move the box, the static friction force is also small and directly opposite to your push. If you push harder, the friction force increases to match the magnitude of your push. There is a limit to the magnitude of static friction, so eventually you may be able to apply a force larger than the maximum static force, and the box will move. The maximum static friction force is sometimes referred to as starting friction. We model static friction, F static, with the inequality F static s N where s is the coefficient of static friction and N the normal force exerted by a surface on the object. The normal force is defined as the perpendicular component of the force exerted by the surface. In this case, the normal force is equal to the weight of the object. Once the box starts to slide, you must continue to exert a force to keep the object moving, or friction will slow it to a stop. The friction acting on the box while it is moving is called kinetic friction. In order to slide the box with a constant velocity, a force equivalent to the force of kinetic friction must be applied. Kinetic friction is sometimes referred to as sliding friction. Both static and kinetic friction depend on the surfaces of the box and the floor, and on how hard the box and floor are pressed together. We model kinetic friction with F kinetic = k N, where k is the coefficient of kinetic friction. In this experiment, you will use a Force Sensor to study static friction and kinetic friction on a wooden block. A Motion Detector will also be used to analyze the kinetic friction acting on a sliding block. OBJECTIVES Use a Dual-Range Force Sensor to measure the force of static friction. Determine the relationship between force of static friction and the weight of an object. Measure the coefficients of static and kinetic friction for a particular block and track. Use a Motion Detector to independently measure the coefficient of kinetic friction and compare it to the previously measured value. Determine if the coefficient of kinetic friction depends on weight. MATERIALS computer Vernier computer interface Logger Pro Vernier Motion Detector Vernier Force Sensor string block of wood with hook balance or scale mass set Physics with Vernier 12-1

11 Computer 12 PRELIMINARY QUESTIONS 1. In pushing a heavy box across the floor, is the force you need to apply to start the box moving greater than, less than, or the same as the force needed to keep the box moving? On what are you basing your choice? 2. How do you think the force of friction is related to the weight of the box? Explain. PROCEDURE Part I Starting Friction 1. Measure the mass of the block and record it in the data table. 2. Connect the Dual-Range Force Sensor to Channel 1 of the interface. Set the range switch on the Force Sensor to 50 N. 3. Open the file 12a Static Kinetic Frict from the Physics with Vernier folder. 4. Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the wooden block. Place a total of 1 kg mass on top of the block, fastened so the masses cannot shift. Practice pulling the block and masses with the Force Sensor using this straight-line motion: Slowly and gently pull horizontally with a small force. Very gradually, taking one full second, increase the force until the block starts to slide, and then keep the block moving at a constant speed for another second. 5. Sketch a graph of force vs. time for the force you felt on your hand. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed. 6. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string. Click to set the Force Sensor to zero. 7. Click to begin collecting data. Pull the block as before, taking care to increase the force gradually. Repeat the process as needed until you have a graph that reflects the desired motion, including pulling the block at constant speed once it begins moving. Print or copy the graph for use in the Analysis portion of this activity. Part II Peak Static Friction and Kinetic Friction In this section, you will measure the peak static friction force and the kinetic friction force as a function of the normal force on the block. In each run, you will pull the block as before, but by changing the masses on the block, you will vary the normal force on the block. Mass Figure 1 Wooden block 12-2 Physics with Vernier

12 Static and Kinetic Friction 8. Remove all masses from the block. 9. Click to begin collecting data and pull as before to gather force vs. time data. 10. Examine the data by clicking the Statistics button,. The maximum value of the force occurs when the block started to slide. Read this value of the maximum force of static friction from the floating box and record the number in your data table. 11. Drag across the region of the graph corresponding to the block moving at constant velocity. Click on the Statistics button again and read the average (or mean) force during the time interval. This force is the magnitude of the kinetic frictional force. 12. Repeat Steps 9-11 for two more measurements and average the results to determine the reliability of your measurements. Record the values in the data table. 13. Add masses totaling 250 g to the block. Repeat Steps 9 12, recording values in the data table. 14. Repeat for additional masses of 500, 750, and 1000 g. Record values in your data table. Part III Kinetic Friction Again In this section, you will measure the coefficient of kinetic friction a second way and compare it to the measurement in Part II. Using the Motion Detector, you can measure the acceleration of the block as it slides to a stop. This acceleration can be determined from the velocity vs. time graph. While sliding, the only force acting on the block in the horizontal direction is that of friction. From the mass of the block and its acceleration, you can find the frictional force and finally, the coefficient of kinetic friction. Wooden block Push Figure Connect the Motion Detector to DIG/SONIC 1 of the Vernier computer interface. If the Motion Detector has a switch, set it to Track. 16. Open the 12b Static Kinetic Frict in the Physics with Computers folder. 17. Place the Motion Detector on the lab table 2 3 m from a block of wood, as shown in Figure 2. Position the Motion Detector so that it will detect the motion of the block as it slides toward the detector. 18. Practice sliding the block toward the Motion Detector so that the block leaves your hand and slides to a stop. Minimize the rotation of the block. After it leaves your hand, the block should slide about 1 m before it stops and it must not come any closer to the Motion Detector than 0.4 m. 19. Click to start collecting data and give the block a push so that it slides toward the Motion Detector. The velocity graph should have a portion with a linearly decreasing section corresponding to the freely sliding motion of the block. Repeat if needed. Physics with Vernier 12-3

13 Computer Select a region of the velocity vs. time graph that shows the decreasing speed of the block. Choose the linear section. The slope of this section of the velocity graph is the acceleration. Drag the mouse over this section and determine the slope by clicking the Linear Fit button,. Record this value of acceleration in your data table. 21. Repeat Steps four more times. 22. Place masses totaling 500 g on the block. Fasten the masses so they will not move. Repeat Steps five times for the block with masses. Record acceleration values in your data table. DATA TABLE Part I Starting Friction Mass of block kg Part II Peak Static Friction and Kinetic Friction Total mass (m) Normal force (N) Peak static friction Average Trial 1 Trial 2 Trial 3 peak static friction (N) Total mass (m) Normal force (N) Kinetic friction Average Trial 1 Trial 2 Trial 3 kinetic friction (N) 12-4 Physics with Vernier

14 Static and Kinetic Friction Part III Kinetic Friction Trial Acceleration (m/s 2 ) Data: Block with no additional mass Kinetic friction force (N) Average coefficient of kinetic friction: k Trial Data: Block with 500 g additional mass Acceleration (m/s 2 ) Kinetic friction force (N) Average coefficient of kinetic friction: k ANALYSIS 1. Sketch of force vs. time graph from Part I. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed. 2. Still using the force vs. time graph you created in Part I, compare the force necessary to keep the block sliding compared to the force necessary to start the slide. How does your answer compare to your answer to question 1 in the Preliminary Questions section? Physics with Vernier 12-5

15 Computer The coefficient of friction is a constant that relates the normal force between two objects (blocks and table) and the force of friction. Based on your graph (Run 1) from Part I, would you expect the coefficient of static friction to be greater than, less than, or the same as the coefficient of kinetic friction? 4. For Part II, calculate the normal force of the table on the block alone and with each combination of added masses. Since the block is on a horizontal surface, the normal force will be equal in magnitude and opposite in direction to the weight of the block and any masses it carries. Fill in the Normal Force entries for both Part II data tables. 5. Plot a graph of the maximum static friction force (vertical axis) vs. the normal force (horizontal axis). Use either Logger Pro or graph paper. 6. Since F maximum static = s N, the slope of this graph is the coefficient of static friction s. Find the numeric value of the slope, including any units. Should a line fitted to these data pass through the origin? 12-6 Physics with Vernier

16 Static and Kinetic Friction 7. In a similar graphical manner, find the coefficient of kinetic friction k. Create a plot of the average kinetic friction forces vs. the normal force. Recall that F kinetic = k N. Should a line fitted to these data pass through the origin? 8. Your data from Part III also allow you to determine k. Draw a free-body diagram for the sliding block. The kinetic friction force can be determined from Newton s second law, or F = ma. From the mass and acceleration, find the friction force for each trial, and enter it in the data table. 9. From the friction force, determine the coefficient of kinetic friction for each trial and enter the values in the data table. Also, calculate an average value for the coefficient of kinetic friction for the block and for the block with added mass. 10. Does the coefficient of kinetic friction depend on speed? Explain, using your experimental data. Physics with Vernier 12-7

17 Computer Does the force of kinetic friction depend on the weight of the block? Explain. 12. Does the coefficient of kinetic friction depend on the weight of the block? 13. Compare your coefficients of kinetic friction determined in Part III to that determined in Part II. Discuss the values. Do you expect them to be the same or different? 12-8 Physics with Vernier