# Acceleration when an object hits the ground

1. Jul 23, 2011

### Uranium235

Ok so this is a problem that's been bothering me ever since the first few days of learning kinematics.

We've been taught that when an object falls, the object has a positive velocity up until the moment that it hits the ground. At that moment, the velocity becomes zero. Wouldn't this imply that the acceleration that the object experiences when hitting the ground is negative infinity since it stops the very moment that it lands?

2. Jul 23, 2011

### Oldfart

The falling object will dent the ground slightly. also the object itself will be deformed a little by the force of the impact. Thus the object does not stop the very moment it lands, it just slows down very rapidly.

3. Jul 23, 2011

### skeptic2

Does it really stop the moment that it lands? Might not both the object and the ground bend a slight amount the moment it touches the ground?

4. Jul 23, 2011

### MisterX

That's not realistically what happens when an object hits the ground. Even if the the ground and the object were rigid, electrostatic forces that prevent solid objects from passing through each other would not result in an such an instantaneous change of velocity. In reality, though, an object and the ground probably would not be perfectly rigid, and the ground would deform downward, a bit like the object was being caught in a net.

5. Jul 23, 2011

### Uranium235

Ok got it! Thanks for the answers guys!

6. Jul 23, 2011

### AlephZero

No real objects are perfectly rigid, so even if there is no permanent change in shape of either the object or the ground, both will be compressed a bit during the time of the impact and then spring back again.

The accelerations when a "rigid" object like a brick falls on the ground can be very large - "shock testing" to check that accidental bumps and falls won't cause damage to objecst is often done at accelerations up to 3000 times the acceleration due to gravity (i.e. about 30,000 m/s2)