Ekins P7A Lab 4

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Apr 3, 2024

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Physics 7A Collisions lab, v. 5.0 p-1of8 LAB 4: COLLISIONS Lab Introduction This lab investigates collisions, both elastic and inelastic. By understanding these different kinds of collisions, you can develop better intuitions, which enables you to solve hard problems and to avoid making some common mistakes involving energy and /or momentum conservation. Equipment Your GSI will show how to use the photogate timers, and the plunger on the launch cart. Experimental Procedure Measuring velocity In the following experiments, we want to know the velocities of the carts both before and after the collisions. We will be using photogates, and a data acquisition program to measure times and calculate speeds. The metal tab on the cart will pass through a single photogate interrupting the light

Physics 7A Collisions lab, v. 5.0 p-20f8 beam. The computer will record the time the beam was interrupted and use that together with the known width of the tab (4 cm) to calculate the average speed of the cart. Note: the photogate doesn't care which way the cart passes through, so we are only finding magnitudes not directions. Also keep in mind this is an approximation (Ax/At) to the instantaneous velocity (dx/dt). NOTE : If you haven't already done so, turn on the computer now. The program you will be using can be started by double clicking on the 7A_Collisions.mbl icon. It is set up to record two times and compute two speeds, one for each photogate. When you click on the START button (or simply hit RETURN) the computer will be ready to collect data (though it only collects it when something passes through the photogates.) You should position the photogates to measure the speeds you are looking for. For example, the figure shown would measure the initial speed of the launch cart and the post collision speed of the second cart. Make sure you pay attention to which cart is passing through which photogate, if you don't it is easy to get mixed up as to which time and speed is which. For example, in the first experiment you will do, the carts will stick together after the collision, that means the tab from cart #2 will pass through photogate #2 followed by the tab from cart #1, i.e. you will have triggered photogate #2 TWICE. A Couple of Suggestions ¢ Make sure the two photogates are set up appropriately to measure the speeds you are looking for and allow enough space for the carts to reach "constant" speeds. E.g. for launch velocity make sure the photogate is not in the region where the cart is accelerating. For the post collision speed, make sure the photogates are after the collision is complete. 4 To launch the cart, set the plunger to its middle (medium) notch. Make sure the compressed plunger is touching the wall. Using a metal bar, hit the plunger release button with a quick, firm vertical stroke. Practice this a few times. With two photogates, you will always be able to compare before and after values for a single collision, but if you want to compare one collision to another, it is useful to try and be consistent in your launching style. 4 Place the carts in the positions you want by picking them up and moving them, if you just slide the carts back along the track, you will trigger the photogates again and get more data that may tend to confuse things. The computer program records data in pairs (time for photogate #1 and time for photogate #2), if you do accidentally trigger one of the photogates, you may have to trigger the other by hand to ready the computer before your next run. Or you can also STOP the data collection and START it again. 1. Without the second cart on the track, launch the first cart several times, letting it pass through BOTH photogates. Record the speeds in your notebook. Are you able to launch the carts reasonably consistently? Is the use of average speed a reasonable approximation in this case? Trial 1: v1=0.05312 v2=0.05256 Yes, we can launch the cart consistently. Because Trial 2: v1=0.05657 v2=0.05691 the speeds are so Trial 3: v1=0.05310 v2=0.05308 consistent, yes it would be reasonable to use an average speed.

Physics 7A Collisions lab, v. 5.0 p-3of8 Inelastic collision In this experiment, the launch cart collides with a stationary 2nd cart sitting near the photogates. By orienting the carts so that their Velcro ends collide, you can make them stick together. The stuck-together carts then pass through the photogates, allowing you to calculate the post-collision velocity, v, Don’t do the experiment until making predictions! 2. (Prediction) Using any technique you'd like, solve for o, in terms of v,. Is v half of vy, or three quarters of v, or what? Both carts have the same mass. That m should cancel out of your answer. Assuming energy is conserved, vf is the square root of half of vO because m2 is 2m1 1/2 m1 vOA2 = 1/2 m2 vir2 m2=2m1 1/2 vON2 = vir2 (1/2)M/2 v0 = vf 4 Making sure the Velcro end of the 2nd cart faces the launch cart, do the experiment. Find v, in terms of v,. Is it (approximately) half of v, three quarters of v;, or what? Data, calculations, and results: V1 =0.737 Vi is approximately half of VO V2 =0.368 V2 =0.49932157 V1

Physics 7A Collisions lab, v. 5.0 p-4of8 3. Was kinetic energy conserved during the collision? Show your work. No, kinetic energy was not conserved during the collision. KEO =/= KEf KEO: 1/2m (0.737)"2 = 0.2715845 m1 KEf: 1/2 m (0.368)"2 = 0.066612 m2 0.2715845 =/= 0.135424 4. How can you reconcile energy conservation with the results of this experiment? The energy was lost from the cart-cart system in the form of friction and in sticking together via velcro.

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