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

[msattar07]

Homework Statement

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

Homework Equations

v= square root of GM/r

The Attempt at a Solution

there's no mass, so i wasnt sure how to solve it exactly

 

[Snazzy]

What does capital M denote?

 

[msattar07]

mass of the Earth

 

[Snazzy]

Well, that's not an unknown mass.

 

[msattar07]

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

the answer is suppose to be B.

 

[Snazzy]

Well, use conservation of energy:

$$U_0 + K_0 = U_f + K_f$$

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