# Collision between moving and stationary object

1. Dec 16, 2012

### Ezio3.1415

Consider a feral elephant that charges a hovering stationary fly with
a speed of 5m/s at a hovering fly. What is the speed with which the fly will recoil?

Well I know the answer's 10m/s as the mass of the fly is negligible with respect to the mass of the elephant... But how?

2. Dec 16, 2012

### mathman

Since the fly mass is negligible, the center of mass is coincident with the elephant. In the c.m. system (stationary elephant) the fly hits the elephant at 5 m/s, bounces off and recoils at 5 m/s in the opposite direction. Add the c. m. speed and the fly will be going 10 m/s.

3. Dec 17, 2012

### Ezio3.1415

Thank you... :)

4. Dec 17, 2012

### sophiecentaur

I prefer the model of a ball bearing being fired at an oncoming express locomotive with a thick steel plate on the front. Same principle but the resulting high speed ball bearing is somehow a more attractive idea than a squashed fly.

Also, a really useful version of this is the way you can use slingshot orbits around large planets to speed up space vehicles. It's all to do with transfer of momentum.