Procedure, cont'd. 

C. Bouncing (Elastic) Collisions

This part of the experiment proceeds much like the first part except that now the spring bumpers on the two gliders should face one another. Since the two gliders do not stick together, they do not have identical velocities after collision and their velocities must be measured separately.

1. If it wasn't so for the sticking collision, make sure that one cart is at least 200g more massive than the other (add a mass to it, for the PASCO carts, add more mass).  Be sure to record this new mass of the cart.
2. Instead of the file "Two Gate Timer.mbl," open the "Collision Timer.mbl" experiment file under File, Open, Probes & Sensors, Photogates.  You can again close the graph part, as it will not be useful. Enter the length of the flag again in Setup, Data Collection, Sampling tab, and be sure the length of the flag on both the carts is the same. 
3. Use the more massive cart as the "target".  Use similar photogate placement as in the Part B.  Set the "target", at rest, just in front of the second photogate so that it will pass through after the collision. The incident cart is started moving from a position near the zero position on the track in front of the first photogate.  The incident cart will move through the first photogate, collide with the target cart, and then the target will pass through the second photogate while the incident bounces back through the first.  Make sure that the "target" goes through the second photogate before the incident glider rebounds through the photogate timer (otherwise the PC will be confused about the order of events). If energy is conserved, the following relationship is valid:

   1/2 m₁v₁² = 1/2 m₁v₁'² + 1/2 m₂v₂'².  

   In this equation m₁ is the mass of the moving glider, m₂ is the mass of the "target" glider, v₁ is the velocity of the incident glider, v₁' is the velocity of the incident glider after the collision and v₂' is the velocity of the "target" glider after the collision.

4. For each collision, first position the gliders, then hit collect, then send them through (be sure to catch them before they hit the ends) and then hit Stop and in the Data menu, Save Latest Run.
5. Note that now you will have three velocity measurements: the velocity of the first cart before the collision and the velocity of each cart after the collision. The velocity of the second cart before the collision (which is 0) is not recorded. You may want to cut and paste these velocities so that all the velocities for one collision are on the same line.
6. Repeat this collision for at least five different initial velocities (with as large a spread as is feasible) and save your data.  It is especially important in the Elastic collision that you don't try to go too fast, as energy will be lost and your data corrupted!  Import the data into the spreadsheet and plot the initial energy versus the final energy.  Do a straight line fit to find the slope and intercept of this graph. Put the best-fit straight line on your graph.
7. Does the slope and y-intercept agree, to within 90% confidence, with the value you expect?

Write-Up

1. If this is a formal lab (as indicated on the lab syllabus), you have been instructed as to whether this is an individual or group write-up, or an oral presentation. If it is a write-up, each person must submit the lab electronically as a word-processed document in Moodle before the next lab meeting. If it is a group write-up, you should all be submitting the same document. For written formal labs, remember to check the "write-up hints" page to be sure everything is included and check your write-up against the grading rubric.
2. If this is an informal lab, work on the results together in your groups, and be sure to have your complete informal lab in your lab notebook and checked by the instructor before you leave.
3. Remember to read the next lab and do the pre-lab before you come to lab next week! You may work on the pre-lab with others, but each person must submit her or his own work.

Department of Physics