Question:

The lazy ball and the bouncy ball. Which one will exert a larger impulse to the “buildring”?

Answer:

This question can be answered easily if one examine the change in momentum of the ball.

For the lazy ball, its momentum is going to change from mv to 0. This means the lazy ball will experience an impulse of mv (rightward). By N3L, it is going to exert an equal but opposite impulse of mv (leftward) on the “building”.

For the bouncy ball, its momentum is going to change from mv to –mv, corresponding to a change of 2mv. This means the lazy ball will experience a stronger impulse of 2mv (rightward). By N3L, it is going to exert a stronger impulse of 2mv (leftward) on the “building”.

Delving Deeper

But didn’t the lazy ball lose more KE? Why didn’t the “building” gain more KE as a result?

The lazy ball did lose more KE compared to the bouncy ball. But most of the KE was converted into heat. It was a completely inelastic collision afterall.

The bouncy ball did lose less KE compared to the lazy ball. But whatever KE it lost was transfered to the “building”. It was a (completely) elastic collision afterall.

Turns out the “building” gained more KE from the elastic collision than the inelastic collision. It was a conclusion that was more easily reached by analysing momentum than KE though.

• Uncategorized