# What was the speed of the projectile when it left the cannon?

1. Apr 7, 2008

### msattar07

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

A projectile is shot from the surface of te Earth by means of a very powerful cannon. If the projectile reaches a height of 35,000 m above Earth's surface, what was the speed of the projectile when it left the cannon?

a. 355 m/s
b. 827 m/s
c. 710 m/s
d. 906 m/s

2. Relevant equations

v= square root of GM/r

3. The attempt at a solution

theres no mass, so i wasnt sure how to solve it exactly

2. Apr 7, 2008

### Snazzy

What does capital M denote?

3. Apr 7, 2008

### msattar07

mass of the Earth

4. Apr 7, 2008

### Snazzy

Well, that's not an unknown mass.

5. Apr 7, 2008

### msattar07

yea so i plugged it in but didnt get the right answer.

the answer is suppose to be B.

6. Apr 7, 2008

### Snazzy

Well, use conservation of energy:

$$U_0 + K_0 = U_f + K_f$$

You know that $$U = -\frac{GmM}{r}$$