# After a glancing collision, why do air hockey pucks spins around in circle?

For a lab testing the conservation of momentum, I had to hit an air hockey puck so that it would hit another stationary puck in a glancing collision. After the pucks collided, they travel their separate ways, but I also noticed that both pucks seem to be spinning in circles as they travel on. This spinning had no effect on movement, as both pucks were only traveling in straight lines throughout the experiment. I did this a few more times and it happened every time I repeated it. What could have caused the pucks to be spinning in circles when all I did was hit one puck straight on, which then collide with the other puck?

• Are you saying the first puck (that collides head on) is spinning, or just the ones the hit at a glancing angle (which would give a tangential friction force and this explain the spinning)? – Floris May 9 '15 at 23:15

## 1 Answer

If the collision is not perfectly along the line connecting the centers of mass of the pucks, they will exert torques on each other as well as forces. The angular momentum of the pair will be conserved, so if the incoming puck was not spinning, the pucks will exit the collision spinning in opposite directions. If the surface they slide on is frictionless, the spinning will not affect the translational motion.

• Wouldn't the linear momentum decrease since angular momentum is generated? – LDC3 May 10 '15 at 3:21
• No, they are different quantities, and are conserved separately. The spinning of an object about its center of mass contributes nothing to the linear momentum. – Bill N May 10 '15 at 3:51
• Where does the energy for the change in angular momentum come from? Since the 2 disks are spinning in opposite directions, then the sum of angular momentum is same, but energy had to come from somewhere to make the change. The only energy I can see is the kinetic energy from the sliding disk. – LDC3 May 10 '15 at 4:13
• That's right. It just like comparing the speeds of a rolling ball, a rolling cylinder, or a frictionless, sliding cube at the bottom of an inclined plane, starting from rest. The cube will have the greatest speed, then the ball, then the cylinder. They all have the same energy, but different speeds because of the rotational motion and moments of inertia. – Bill N May 10 '15 at 13:39
• I would still like to know where the energy came from to get the disks spinning. – LDC3 May 10 '15 at 13:47