PHY 221 Lab 5 Diverse Forces, Springs and Friction

Transcription

1 Name: Partner: Partner: PHY 221 Lab 5 Diverse Forces, Springs and Friction Goals: To explore the nature of forces and the variety of ways in which they can be produced. Characterize the nature of springs and to investigate friction. Materials: Computer-based measurement system cart and two wireless force probes (or one wireless and one wired) Spring scales Masses A variety of force-producing systems. See PRELAB page 11. Activity: 1. Exploring and characterizing forces You are already very familiar with some forces from last week s lab. However, there are many kinds of forces. There are a number of different force-generating devices or systems that you can bring back to your station to study (see the Appendix attached at the end). Be sure that your choices are as dissimilar as possible and do not include springs as they are reserved for activity 2. Your team should bring back one at a time, then fill out the lab sheet according to these instructions: 1

2 a. Describe the kind of force. b. How strong is the force? Can it reach hundreds of Newtons? A couple of Newtons? A fraction of a Newton? Less? You can use spring scales of different range or computerized force probe to explore magnitude of the force. Use small coil spring to probe the weakest forces (first see how much force is needed to stretch this coil out). c. What does that strength depend on? (for example: distance between something and another thing, velocity, mass, electric charge, weight, chemistry, amount of squeeze/stretch,...) Does the force depend on contact between things? d. What is the direction of the force? (for example: attractive, repulsive, against direction of motion, upwards, downwards, outwards,...) Note that by asking you to give the magnitude and direction of the force, we are asking you to characterize a vector. 1) Kind of force: Strength of force: What strength depends on: Direction of force: 2) Kind of force: Strength of force: What strength depends on: 2

3 Direction of force: 3) Kind of force: Strength of force: What strength depends on: Direction of force: 4) Kind of force: Strength of force: What strength depends on: Direction of force: 3

4 2. Springs We all think of springs as coils of metal. A more general way to think of a spring is as something that changes the location of one of its ends when you pull or push on it, by an amount that depends on how hard you push or pull. Springs of the ordinary coil-shaped sort are at the heart of your spring scale. Here we want to explore other kinds of springs. For each spring, give a description, state whether it works in tension (by stretching) or in compression (by squeezing) or transversely (by moving sideways), and measure how far its end moves for a given force. Check whether it moves by twice as much when you give twice as large a force, and that it comes back to its original state when you remove all of the external forces. A spring that does this is said to obey Hooke's Law. As part of the characterization below since you have a force probe and you have a ruler you should plot an extension-force curve for each spring and plot it on the computer. To set this up go to Experiment/Data Collection. Chose Events with entry. Enter Column Name as extension distance short name distance. Click on Collect. Click on the Keep symbol (the iris diaphragm) which is to the right of the green Collect button when the spring extension is at the desired extension distance. Then the force at that distance will be automatically entered associated with the extension distance that you will type in from reading the ruler following each Keep action. In this way collect all the needed values. Be sure to enter into the computer the actual distances in cm. Your instructor will show you the variety of springs to study. Spring 1 (tension) description and characterization as above: 4

5 Spring 2 (compression) description and characterization (Make an additional column and plot as an additional curve): Spring 3 (constant force) description and characterization: (Make an additional column in your data table and plot as an additional curve): PRINT all three extension force curves 3. Friction Use the cart with no mounting bracket. Unlike previous uses of the cart, this time turn the cart upside down, so that the wheels are pointing upwards. Put two rectangular weights on top of the cart to increase its mass. Tie a piece of string to the hook on the force probe, and tie the other end to a convenient spot at one end of the cart. Zero the tethered Force Probe or the Wireless Force Probe. Get ready to move the cart by pulling on the force probe. The string should be stretched but no detectable force should be applied to it. Start collecting the data with the cart at rest. Start applying the force. Do it very gently so you can explore range of forces 5

6 for which the cart is still at rest. If you can, make the force increase linearly with time. Once the cart starts moving try to maintain constant force and constant speed of the cart. Then suddenly increase the pulling force and make the object accelerate. Since this is going to be printed out, you may wish to repeat this experiment until it looks good. PRINT all four graphs to add to this report. Underline the print out indicate 1, 2, 3, 4 and 5 second marks on the time axes for each plot. When reading your graphs pay attention to where the zero on vertical axis is, and draw on your print out the moment or point at which there was a change of trend in the measured quantity. Identify on the print out of the graphs when the force was already applied but the cart was still at rest. Indicate this range by vertical bars on the force vs. time graph and label it Region 1. Mark the same time period on the other three graphs. The force we measured here is clearly not zero, but since the cart is at rest it has zero velocity and acceleration. Doesn t this contradict the F = ma law? Explain. What force F do you use in the equation F = ma? The new force, which came into play here, is called static friction. Draw a free body diagram for the cart and indicate the static friction force, fs, and the force exerted by the string on the cart, T. Which force is measured by the force probe? 6

7 Does static friction have a constant magnitude? Can static friction assume any value? Now on the print out of your velocity vs. time graph, indicate when the cart was moving with approximately constant velocity. Mark this time period with vertical bars and label it Region 2. Mark the same time interval on the other three graphs. Since velocity is approximately constant, the position vs. time graph should be linear and acceleration should be approximately zero. Check it with the computer. Again, the measured force is not zero but there is no acceleration. The force exerted on the cart, T, is balanced by the force of kinetic friction, fk. Unlike static friction force, kinetic friction force does have a constant magnitude independent of the force it opposes. Thus when T is increased fk cannot balance it and the object accelerates. Identify time interval on your graphs that corresponds to this of motion and mark it Region 3. We will now quantitatively investigate the force of kinetic friction. We can obtain magnitude of this force from the Region 2, since here, T = fk. Stretch a selection range on the force vs. time graph to cover the Region 2. Then go to Analyze menu and click on Statistics. Make sure that the selection bars don t disappear when you do that. Store the mean value of the force in this range (displayed in the superimposed box) in the table below. Remove one rectangular weight from the cart. Zero the force probe. Repeat the experiment. This time you just need to concentrate on obtaining motion with constant velocity ( Region 2 ). Stretch the selection range on the force vs. time graph for the Region 2, and obtain mean force value in this time interval. Store it in the table. Finally, take the second weight off and repeat the measurement process for the cart alone. Remember to zero the force probe before taking data. 7

8 Object Cart with two rectangular weights fk (N) Mass measured with a balance - m (kg) Coefficient of kinetic friction Cart with one rectangular weight Cart by itself Measure mass of the cart with and without weights using a balance and store your measurements in the table. Sketch here a graph of fk vs m. What kind of simple function could describe dependence of fk on m? 8

9 Complete the table by calculating coefficient of kinetic friction: k = fk / N. Here N is a normal force exerted by the track on the cart. Since the cart is not moving in vertical direction this force balances the weight of the cart: N=mg. (Include N and mg on your free body diagram). This coefficient should not depend on the force applied to the cart, mass of the object, nor the area of the surfaces that are in touch. It does depend on type of the surfaces that are at friction. Do the results for the coefficient roughly agree between the three measurements? Since static friction is not a constant force, coefficient of static friction is not defined as s = fs / N. You may have noticed that the maximal value of the static friction decreases with the mass of the object. In fact, coefficient of static friction is defined as s =(maximal fs) / N. Coefficients of static and kinetic friction are usually similar but not identical. It is obvious that if they are not equal then, s k. This can be observed as slight drop in the measured force as you transit from Region 1 (object at rest) to Region 2 (motion with constant velocity). Do you see the effect on the force vs. time graph that you copied to your report? If you don t try to take data again (cart with two weights on) concentrating on the moment when the cart starts moving, don t apply more force than needed to keep it in motion. Invert the cart and put it back on its wheels with two weights on top. Measure coefficient of kinetic friction. Show your result here, and compare it to the value obtained before. 4. Forces between a pushed mass (your chance to design your own experiment) Suppose you had two carts on a frictionless track, one with two rectangular weights attached and one without added weights. Suppose these carts were placed together and you pushed one of them with a set force. What would you expect the force to be 9

10 from the first cart to the second if the heavier cart (m1) was being pushed with a force, F, and if the lighter cart (m2) was being pushed with the same force? Would the inter-cart force be F or some other value? What would the force be from the second cart to the first if the heavier cart (m1) was being pushed with a force, F, and if the lighter cart (m2) was being pushed with the same force? Try it. Put a Wireless Force Probe on the lighter cart. Use the tethered Force Probe to push the first cart with a measured force, F. Do an experiment to test your expectation and describe it below. Create the appropriate graph of results and PRINT it out. Appendix to PHY 221 Lab 5: Examples of Forces: 1. Muscles two opposed sets since force can only be applied with shortening the muscle 2. Weights 3. Tension in strings or ropes 4. Force between two magnets 5. Electrostatic force (two balloons, charged by rubbing on hair or on sweater) 6. Buoyancy (boat floating in water) 7. Air pressure (air pump and bicycle tire) 8. Air exhaust rocket (balloon) 9. Chemical bonds between layers of adhesive tape 10. Sliding friction 11. Air friction 12. Fluid friction 13. Friction from magnet-induced currents ( eddy currents ) 10

11 Name: Lab 5 prelab Two blocks are in contact on a frictionless table. A horizontal force is applied to the m1 block as shown in figure below. What is the force of contact between the two blocks? What would the force of contact be if the horizontal force were applied to m2 rather than m1? F Is the force of contact between the two masses different? Why or why not? 11