# Gravity - Universal question

1. Apr 18, 2009

### cash.money

1. The problem statement, all variables and given/known data
An object of mass 40.0 kg rests on the surface of a planet with a mass of 8.2 x 1022 kg and radius 3.6 x 105 m.

a) calculate the force of gravity acting on the object.
b) Determine the gravitational field strength "g" at the planet surface.
c) Calculate the force of gravity acting on the object if it is placed at a position 6.4 x 105 m above the planet's surface.

2. Relevant equations

Newton's law of universal gravitation:

F=G m1m2/r2
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 18, 2009

### whybother

You can answer (1) and (3) right? You've written the correct formula, you just need to make sure for (3) you add the radius of the planet to the height the object is above the planet.

And you know $$F=ma$$, or in this case, $$F=mg$$... so there should be (2)

3. Apr 18, 2009

### ken~flo

plug in your #'s for (a) to get F, set that equal to mg for (b), where g is what ur tryin to find. (c) is just # crunching with a different radius

4. Apr 18, 2009

### cash.money

But what is G supposed to equal. Is it G = 6.673 × 10-11 N

and by crunching you mean replace the old radius with the new one right?

5. Apr 18, 2009

### Stratosphere

You will also need

$$g=\frac{GM}{r^{2}}$$

6. Apr 18, 2009

### whybother

That's not a unique formula. That's just combining the expression given for F with F=ma.

7. Apr 19, 2009

### ken~flo

yeah, you have the right value for G. and for c, you're right, all you do is plug in the new radius