# Sun's gravitational force

1. Apr 8, 2010

### moonman239

This book that I'm reading mentions something where the gravitational pull of the Sun is proportional to that planet's mass/its orbital radius. What is this proportionality constant?

2. Apr 8, 2010

### mgb_phys

Not quite , for an object in orbit the centrifugal force throwing it outwards is equal to the gravitational force pulling it into the object in the centre.

3. Apr 8, 2010

### moonman239

Oh, okay. So really, F=(4pi^2mr)/T2 should be used.

4. Apr 8, 2010

### Janus

Staff Emeritus
The gravitational pull on any planet by the Sun is proportional to that planet's mass divided by the square of its orbital radius. The actual formula is

$$F_g = G\frac{M_{sun}M_{planet}}{r_{orbit}^2}$$

where G is the gravitational constant and is equal to 6.673e-11 when M is measured in Kg and r in meters.