# Physics Java Labs

Before you start your lab report, it might be useful to look at a sample report.

## Lab: Collisions and Conservation Laws

• Purpose of experiment:
In collisions, the total momentum is conserved. In totally eleastic collisions, the total kinetic energy is conserved in addition. We will measure the ratio of the two masses of the two colliding carts
m1 / m2

in each collision, based on our knowledge of these conservation laws, in particular momentum conservation.

• Video clips of experiment: (each about 0.2 MB)
Collision 1
Collision 2
Collision 3
• Instructions for performing the experiment:
1. Run the java applet by clicking one of these buttons:

(They will open in a separate window, and you will have to do each of the three experiments to find the mass ratios).
2. Digitize the motion of both carts by clicking on each of them. After each pair of clicks, the mouse position is recorded and the movie is advanced one frame. You should always try to click on the same point of the carts - we recommend the right edge of the cart on the left (cart 1) and the left edge of the cart on the right (cart 2). If you make a mistake, click on the "Undo Pt." button in the applet, and the last point will be erased.
3. You can display the points that you have already digitized before by clicking on the "Plot y(x)" button. It will turn red and show the points overlaid on the video. Clicking the same button again will turn this feature off.
4. You can also plot the trajectory as a function of time directly within the applet by clicking on the "Plot x(t)" button. Clicking on the "Digitize" button will bring you back to the data input mode.
5. The numerical data you have collected are displayed in the text area on the right side of the applet. After you are done digitizing all frames, produce a plot with your graphing program or by screen-capture of the applet.
6. Numerical work:
• You need to measure the initial and final velocities of each cart. You can do this graphically by measuring the slopes of the lines, or by using the numbers in the columns on the right.
Please note: If you would like some help on how to do this, here is a movie you can look at: big version (280 kB) or small version (90 kB)
• Each velocitiy is given by
v = Dx / Dt = (x2 - x1) / (t2 - t1)
• Once you have the velocities, you can use the momentum conservation equation,
m1 v1 + m2 v2 = m1 v'1 + m2 v'2

to extract the ratio m1 / m2.

7. Finally, write the report with your favorite word processor or text editor and email it (as email attachment) back to the instructor.