Motion querey

[Rattle Bag]

A truck is travelling towards me at ..say..50kmph. I throw a stone at it, it hits the truck and bounces off back towards me. The stone's speed has gone from a positive to a negative value, passing thru zero along the way. If the position of the stone at the instant it hits zero is in contact with the truck, is the trucks speed also zero at this instant?

[Rev. Green]

No, the stone will be in contact with the truck for a small, but finite, time. During this time the stone and truck both deform slightly. When the stone is stationary the truck will still be moving and both will be distorting - the small part of the truck in contact with the stone will be moving at the same speed as the part of the stone in contact with the truck - which will be close to zero.