# Power of earth

1. Nov 17, 2013

### Hepic

If we let a thing from a height,earth force that with F=m*a=m*g=B(weight).
BUT What is the power of earth,when the thing goes down(the power that force the thing at earth too).I mean the different power,that keeps that in the ground. Logically the force of earth,is about the speed of thing,but what is the law?

Thank you!!!

2. Nov 17, 2013

### CWatters

Not sure if I understand your question.

If you drop something it will accelerate towards the ground with a = g.

When it hits the ground it will penetrate a distance "s". You could calculate the acceleration (deceleration) that occurs when it hits the ground using a SUVAT equation such as..

V2 = U2 +2as

where

V is the final velocity = 0
U is the initial velocity (eg at impact with the ground)
S is the stopping distance (the distance it penetrates the ground)
a is the acceleration.

Then solve for "a"

More here. http://en.wikipedia.org/wiki/Equations_of_motion

If you want to know the average force during the impact you could apply F=ma.

3. Nov 17, 2013

### tiny-tim

Hi Hepic!

I'll just add this to what CWatters has said …
No, force is about change of speed …

the same force (applied for the same time) will increase the speed from 0 to 1, or from 100 to 101.

So the force is the same for a moving thing as for a stationary thing.

(and of course when something is on the ground, there are two forces on it, the weight mg down and the normal force mg up … so the total force is 0, and the change in speed is 0!)

4. Nov 17, 2013

I too didn't understand your question.
The force of Earth you are talking about is Gravity.Gravity exists between all masses and is given by the formula:

$F=\frac{Gm1m2}{r2}$

http://en.wikipedia.org/wiki/Gravitation

Last edited: Nov 17, 2013
5. Nov 18, 2013

### tiny-tim

you can't use the SUB tags inside latex!

6. Nov 18, 2013

Then how do I write that equation?

EDIT:test,
$F=\frac{Gm_1m_2}{r^2}$

Last edited: Nov 18, 2013
7. Nov 18, 2013

### tiny-tim

F=\frac{Gm_1m_2}{r^2} …

$F=\frac{Gm_1m_2}{r^2}$