# Escape speed

1. Mar 26, 2007

### imy786

1. The problem statement, all variables and given/known data

By considering the gravitational potential energy of an object at the surface , find an expression for the escape speed from the surface and determine its value.

2. Relevant equations

GPE= mgh

3. The attempt at a solution

KE= 1/2* m * v^2

GPE= mgh

GPE+ KE = 0

mgh = - 1/2 * m * v^2
gh= - 1/2 * v^2

v^2= -2gh

v= sqare root of (-2gh)

this is the escape velocity...

2. Mar 26, 2007

### Dick

You've computed velocity needed to reach a height h in a uniform gravitational field. Is that the same concept as 'escape velocity'?

3. Mar 26, 2007

### HallsofIvy

You also appear to be assuming gravitational force is constant. If that were true, there would be no "escape velocity". It is impossible to "escape" a constant force!

4. Mar 26, 2007

### anantchowdhary

Well if ur looking for escape velocity....it should be$$\sqrt{\frac{2GM}{R}}$$

5. Mar 26, 2007

### imy786

thanks for the info...
so how shall i start off instead

6. Mar 26, 2007

### anantchowdhary

you shud use the work energy theorem

Last edited: Mar 26, 2007
7. Mar 26, 2007

### Dick

You should change your expression for gravitational potential energy to one appropriate for a nonconstant field.

8. Mar 26, 2007

### HallsofIvy

Remember that gravity is an "inverse square formula"!