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

1. 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

2. Homework 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

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What does capital M denote?

mass of the Earth

Well, that's not an unknown mass.

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

the answer is suppose to be B.

Well, use conservation of energy:

$$U_0 + K_0 = U_f + K_f$$

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