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

m_{1} / m_{2}

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

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).

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.

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.

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.

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.

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 = (x_{2} -
x_{1}) / (t_{2} -
t_{1})

Once you have the velocities, you can use the
momentum conservation equation,