# Gravity - Universal question

## Homework Statement

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.

## Homework Equations

Newton's law of universal gravitation:

F=G m1m2/r2

## The Attempt at a Solution

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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)

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

plug in your #'s for (a) to get F
But what is G supposed to equal. Is it G = 6.673 × 10-11 N

(c) is just # crunching with a different radius
and by crunching you mean replace the old radius with the new one right?

## Homework Statement

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. Homework Equations

Newton's law of universal gravitation:

F=G m1m2/r2
You will also need

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

You will also need

$$g=\frac{GM}{r^{2}}$$
That's not a unique formula. That's just combining the expression given for F with F=ma.

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