# Homework Help: How high above the the earth's surface is the meteor?

1. Jul 20, 2011

### fa08ti

a 12 kg meteor experiences an acceleration if 7.2 m/s^2. when falling towards the earth

a: how high above the the earth's surface is the meteor?
b: what force will a 30 kg meteor experience at the same altitude?

attempt:

i'm, not sure which equation to use..would it be
v= sqrt(Gm/r)?

2. Jul 20, 2011

### Pi-Bond

Re: meteor

You need a formula which relates acceleration due to the earth's gravity and the distance from it's center. Do you know such a formula?

3. Jul 20, 2011

### fa08ti

Re: meteor

would this be correct?

gh = GM / (R + h )2

4. Jul 20, 2011

### Pi-Bond

Re: meteor

If R is the radius of the earth, and h above the surface of the earth, yes.

$g=\frac{GM}{(R+h)^{2}}$

5. Jul 20, 2011

### fa08ti

Re: meteor

i used the formula and got a negative value?

Last edited: Jul 20, 2011
6. Jul 20, 2011

### Dick

Re: meteor

How did you get a negative value? g should come out to 9.8m/s^2 if h=0. Do you know why? To get 7.2m/s^2, h should certainly be positive.

7. Jul 20, 2011

### fa08ti

Re: meteor

i used the mass of the meteor and i think i should have used the mass of earth?

8. Jul 20, 2011

### Dick

Re: meteor

Very correct.

9. Jul 20, 2011

### fa08ti

Re: meteor

thanks..i just want someone to clarify something..does the mass of the object not matter? if so, why not?

10. Jul 20, 2011

### Staff: Mentor

Re: meteor

The mass of the object does not matter. Every mass falls with the same acceleration (in a given gravitational field).

$$F = G\frac{M m}{r^2}$$
but $F = m a$ so that $a = F/m$ Thus

$$a = \frac{F}{m} = G\frac{M}{r^2}$$

Only the mass of the Earth, M, matters for the acceleration of mass m in its field (that's the acceleration with respect to the Earth, of course).

So, why should this be so? It is so because inertial mass happens to be equal to gravitational mass for any object with mass (Look up "equivalence principle").

11. Jul 20, 2011

### fa08ti

Re: meteor

thanks everyone sooo much