Conservation of Momentum Photographed……

During my first year of college I took a physics class where we studied kinematics. The picture you see was taken during one of my physics labs and was used to get data points. It shows an air-hockey table and a puck that is attached to a center pin and rubber band. As the picture was being shot, I gently swung the puck so that it would follow the elliptical path photographed. The lighting for this picture utilized a strobe light, thus we see what appears to be twelve pucks when there is only one.

In our experiment, we wished to witness and record conservation of angular momentum which is also commonly referred to as Kepler's second law, which, when reworded a bit, basically states:

The puck moves so that if an imaginary line were drawn from the pin to the puck, the puck sweeps out equal areas in equal periods of time.

This has the effect of causing the puck to move fast when it is closer to the pin and slower when it is further away from the pin thereby keeping the areas equal. An average speed of the puck was detected by this experiment because the strobe light used flashed at a constant rate. The spacing of each puck in the picture represents an equal interval of time. If you look closely you can see that the puck is, indeed, moving faster when it is closer to the pin (the spacing is further apart)**. From the photograph we estimated the area swept by the puck during these intervals of time and found it to be constant (within our experimental uncertainty).

 ** The spacing at the very top of the photograph was not included because the puck never traveled between those two points.