# Hi Our teacher told us the following:F = m * v² / r = m * 4 pi² /

1. Oct 18, 2011

### physwha

hi

Our teacher told us the following:

F = m * v² / r = m * 4 pi² / T² * r

= m * 4 pi² / k * 1 / r²

= c * m / r²

F = y * mM / r2

Why is there the k and 1 / r² and later the c?

2. Oct 18, 2011

### vela

Staff Emeritus
Re: Gravitation

Did this have to do with one object orbiting another?

3. Oct 18, 2011

### physwha

Re: Gravitation

Yes, it's about the Earth around the Sun for example.

PS: I now understand the c but still not k (is it the same?) and 1 / r² :S

4. Oct 18, 2011

### vela

Staff Emeritus
Re: Gravitation

Are you familiar with the following formula?
$$T = 2\pi\sqrt{\frac{r^3}{GM}}$$where T is the time taken for one orbit, r is the radius of the orbit, G is the gravitational constant, and M is the mass of the body being orbited.