Objective

1. Make a video of two colliding spheres
2. Analyze the motion of the spheres before and after collision
3. Determine whether momentum and kinetic energy are conserved.

Resources

• Phone or digital camera
  • High-speed Motion Capture with Casio Exilim Camera
• Tracker Software
  • Quick Tutorial on Tracker
• Smooth plastic or metal spheres
• Short length of angle iron (makes aiming the steel sphere easier)

Background

Momentum, the product of mass and velocity, has the property that it is always conserved as long as no outside forces act on the system being considered. It is a vector quantity, so the components of momentum in each coordinate direction are also conserved.

Method

Since the objective is to show that momentum is conserved in a 2D collision, you should create a collision that is not simply 1-dimensional. Try beginning with one sphere stationary on the floor, then hit it off-center with the second sphere so that both travel off at an angle to the striking sphere’s original velocity. Because rolling is involved in this collision, avoid collisions where the initially-moving sphere rebounds backward, as it will backspin and lose a lot of energy to friction.

You can use the piece of angle iron (the long piece of aluminum with an L-shaped profile) as a gentle ramp to roll the striking sphere into the stationary sphere. It’s a little bit easier to aim the sphere this way, rather than rolling it with your hand. Leave some space between the end of the ramp and the target sphere in your video, so you have enough frames to determine the sphere’s initial velocity.