# Hitting the ground

Tags:
1. Nov 16, 2015

### Phys_Boi

If you fall off of a chair you don't get hurt, however, if you fall off of a building you die. How is this possible if you are experiencing the same force..
F=mg

2. Nov 16, 2015

### Staff: Mentor

As the old joke goes, it is not falling down that hurts, it when you stop falling!

The force that will hurt you is the one due to the deceleration, (negative acceleration) when you suddenly stop. The faster you were going, the greater the deceleration.

3. Nov 16, 2015

### Phys_Boi

Okay! So how do you know the time from the initial velocity and 0 velocity?

4. Nov 16, 2015

### Staff: Mentor

There is no easy way to calculate that. It depends on so many things, like the softness of the ground and how much deformation you can sustain before breaking. But generally speaking, you can consider that the stopping time is about the same in all cases, and therefore that the higher the velocity, the stronger the force.

It is probably more relevant to consider it from the point of view of kinetic energy rather than force. The more kinetic energy you have, the more will have get dissipated quickly when you hit the ground, so the more of that energy will go into squashing you and breaking your bones.

5. Nov 16, 2015

### Phys_Boi

Okay! Thanks!