# How much would the earth recoil or move if (energy/collision?)

1. Oct 21, 2007

### gills

How much would the earth "recoil" or move if...(energy/collision?)

1. The problem statement, all variables and given/known data
all 6.5 billion people in the world with an average mass of 70kg, simultaneously jumped off of a 4 meter high platform. What energy would be imparted on the earth assuming people don't bend their knees when landing?

2. Relevant equations
E_f = E_i

and possibly inelastic collision assuming that people keep their knees locked when landing and bones don't break!:

m1v1 + m2v2 = (m1 + m2)Vf

3. The attempt at a solution

I can do the energy imparted part--->

(1/2)mv^2 = mgh to find velocity of people then plug v into -->

(1/2)mv^2 to find the Energy that would imparted on earth.

I'm having a tough time figuring out how much the earth would actually "recoil" or move?? Will it even move at all?

Last edited: Oct 21, 2007
2. Oct 21, 2007

### MrXow

The amount that the earth recoils or move depends on where the energy is imparted to the earth. For example, however unlikely, if everyone did this jump in the state of Kentucky, the earth would seemingly "recoil" in the opposite direction. However, the entire population of the world does not live in Kentucky the population is more or less distributed evenly, so the recoil would probably not be seen.