7.2

Collisions in One Dimension

Student Notes

This simulation shows a 1-dimensional collision between two objects. The elasticity determines the type of collision: 0 for a completely inelastic collision; between 0 and 1 for an inelastic collision; 1 for elastic, and >1 for superelastic, in which kinetic energy is added to the system at the point of collision. The small green dot represents the center-of-mass of the two balls.
Some things to investigate include:

1. | Is momentum conserved in all cases? See if you can find a combination of masses, initial speeds, and elasticity that will produce a total momentum after the collision that is different from the total momentum beforehand. What must you conclude from this?
2. | Under what circumstances is kinetic energy conserved?
3. | Pay close attention to the motion of the center-of-mass during the collision. In general, the motion of the individual balls is changed by the collision. Is the same thing true for the center-of-mass? Explain this.

Self-test

1. | Can one of the objects lose momentum in a collision? In a collision between two objects, where must that momentum end up?
2. | An elastic collision between objects in which one is three times heavier than the other is a special case. Set this up with the two objects having equal speeds. What happens after the collision? This is often done as an in-class demonstration, placing a baseball or tennis ball on top of a basketball and dropping them together. Try this yourself to see if the predictions of the simulation reflect reality. Be careful to avoid the top ball after the collision!
3. | The simulation allows you to consider superelastic collisions, in which the total energy is larger after the collision. Where could this kinetic energy have come from?

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