Lab Force and Motion

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University of Maryland, Baltimore County *

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111

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Physics

Date

Dec 6, 2023

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docx

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Save your work! Name_______________________________________________Section______________________ LAB: FORCE AND MOTION Learning goals:  C l e a r l y descri b e e xp erime ntal o b serv at io n s i n w ords .  Give a p hysic al i nt er p re tat io n (i . e ., e xpla i n t he me an i ng of) th e fu nct iona l re lat io n shi p s an d co n s tant s i n a mat he mat ic al mode l of e xp erime ntal d ata.  Inf er selected phys ical law s fro m direc t e xp erime ntat ion.  Identify forces acting on objects and construct free - ‐ body diagrams.  Carry out proportional reasoning with Newton’s second law.  Reason qualitatively with Newton’s second law; i.e., infer magnitudes of forces or the motion of an object. Materials:  computer-based laboratory system  RealTime Physics Mechanics experiment configuration files  force probe  motion detector  spring scale with a maximum reading of 5 N  track  cart  500 g cube mass to attach to cart  low-friction pulley, lightweight string, table clamp, variety of hanging masses In the previous labs, you have used a motion detector to display position-time, velocity-time, and acceleration-time graphs of different motions of various objects. You were not concerned about how you got the objects to move, i.e., what forces (pushes or pulls) acted on the objects. From your own experiences, you know that force and motion are related in some way. To start your bicycle moving, you must apply a force to the pedal. To start up your car, you must step on the accelerator to get the engine to apply a force to the road through the tires. But exactly how is force related to the quantities you used in the previous lab to describe motion-- position, velocity, and acceleration? In this lab you will pay attention to forces and how they affect motion. You will learn how to measure forces. By applying forces to a cart and observing the nature of its resulting motion graphically with a motion detector, you will come to understand the effects of forces on motion. Note: Since forces are detected by the computer system as changes in an electronic signal, it is important to have the computer "read" the signal when the force probe has no force pushing or pulling on it. This process is called "zeroing." The electronic signal from the force probe can change slightly from time to 1

Save your work! time as the temperature changes. Therefore, zero your force probe with nothing attached to the probe before making each measurement. Investigation 1: motion and force You can use the force probe to apply known forces to an object. You can also use the motion detector, as in the previous two labs, to examine the motion of the object. In this way you will be able to explore the relationship between motion and force. Activity 1-1: Pushing and Pulling a Cart In this activity you will move a low friction cart by pushing and pulling it with your hand. You will measure the force, velocity, and acceleration. Then you will be able to look for mathematical relationships between the applied force, the velocity, and acceleration, to see whether either is (are) related to the force. 1. Set up the cart, force probe, and motion detector on a smooth level surface as shown below. The mass of the cart should be about 1 kg including the force probe and a 500 g cube mass. Make sure the motion detector sees only the cart and not the cable connecting the force probe to the computer. Prediction 1-1: Suppose you grasp the force probe hook and move the cart forward and backward in front of the motion detector. Do you think that either the velocity or the acceleration graph will look like the force graph? Is either of these motion quantities related to force? (That is to say, if you apply a changing force to the cart, will the velocity or acceleration change in the same way as the force?) Explain. The force graph will look like the acceleration graph. This is because when you are moving the motions back and forth, you have to reduce the force to slow it down and then add more force to put it in the opposite direction. The velocity graphs will most likely be a parabola which will not look like the motion of the force and velocity. F=ma. 2

Save your work! 2. To test your predictions, open the experiment file called Motion and Force (L03A2-1) . This will set up velocity, force, and acceleration axes with a convenient time scale of 5 s, as shown below. 3. Zero the force probe. Grasp the force probe hook and begin graphing. When you hear the clicks, pull the cart quickly away from the motion detector and stop it quickly. Then push it quickly back toward the motion detector and again stop it quickly. Try to get sudden starts and stops, and to pull and push the force probe hook along a straight line parallel to the ramp. Do not twist the hook. Be sure that the cart never gets too close to the motion detector. Be sure your hand and body are out of the way of the motion detector. 4. Insert your velocity-time, force-time, and acceleration-time graphs on the next page. 3

Save your work! Question 1-1: Does either graph-velocity or acceleration-resemble the force graph? Which one? Explain how you reached this conclusion . The acceleration graph has a partial resemblance of the force graph. The force graph appears as a solid straight line, with occasional peaks and vallies at the same points where acceleration peaks and vallies. The maginitude of these are a lot smaller than the acceleration graph. The velocity had no resembelence to the graph of force. Question 1-2: Based on your observations, does it appear that there is a math e matical relationship between either applied force and velocity, applied force and acceleration, both, or neither? Explain. Yes, the force and acceleration are directly related, meaning as acceleration increases, force will also increase. This is because as the acceleration was positive and increasing, so was the force at the same time. Activity 1-2: Speeding Up Again You have seen in the previous activity that force and acceleration seem to be related. But just what is the relationship between force and acceleration? 1. Set up the ramp , pulley , cart, string, motion detector , and force probe as shown below . The cart should be the same mass as before (abo ut 1 kg). Be sure that the cart's friction is minimal. 4

Save your work! 2. Prepare to graph velocity, acceleration, and force. Open the experiment file called Speeding Up Again (L03A2-2) to display the velocity, acceleration, and force axes that follow. Prediction 1-2: Suppose that you have a cart with very little friction and you pull this cart with a constant force as shown below on the force-time graph. Sketch your prediction in Logger Pro on the axes if the force graph looks like the one below . 5

Save your work! 3. It is important to choose the amount of the falling mas s so the cart doesn't move too fast to observe the motion. Experiment with different hanging masses until you can get the cart to mo ve acro ss the ramp in about 2-3 s af ter th e mass is release d. Try starting with about 20 g of hanging mass. Record the hanging mass that you decided to u se: Also te s t to be sure that the motion d e tector sees the car t during its complete motion. Remember that the back of the cart must always be at l e a s t 0.5 m from the motion detector. 4. Zero the force probe with the string hanging loosely so that no force is applied to the probe. Zero it again befor e eac h graph. 5. Begin graphing. Release the cart after you hear the clicks of the motion detector. Be sure that th e cab le from the force probe is n ot seen by the motion d e t ec t o r , and that it doesn't dra g or pull the cart. Repeat until you get good graphs in which the cart is seen by th e motion det ecto r over its whole motion. 6. Transfer your data so that the graphs will be persistently displayed on the screen. 7. If necessary, adjust the axes to disp l ay the graphs more clearly. Insert your graphs along with your prediction on the next page. 6

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