# Physics gravitational questioonneed help

1. Apr 7, 2010

### crystal5428

I need a help.

Find the point where the gravitational force from the earth is 2 times of that from moon.

2. Apr 7, 2010

### Andrew Mason

What is the force of gravity at a point Re from the centre of the earth? What is the force of gravity at a point Rm from the centre of the moon? What is Rm at a point on a line between the centres of the earth and moon that is Re from the centre of the earth?

AM

3. Apr 7, 2010

### crystal5428

only the mass of earth, mass of moon and radius of moon and earth is given.
mass of earth=5.98*10(to the power of 24)
mass of moon=7.35*10(to the power of 22)
radius of earth and moon=3.84*10(to the power of 8)

4. Apr 7, 2010

### Andrew Mason

The distance between the centres of the earth and moon is 3.84 x 10^8 m or 3.84 x 10^5 km.

Just use Newton's law of universal gravitation to express the force of gravity from the earth at a distance R from the centre of the earth (R>radius of the earth). Same for the moon. Then set the force of gravity of the earth equal to twice the force of gravity of the moon.

AM