# Finding Velocity with Kinetic Energy.

1. Apr 21, 2013

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

I have to use energy conservation to solve this. So i cannot solve with kinematics

"On a windless day, the engine of a 250 g toy rocket exerts 30N of thrust straight upward until it burns out at a height of 165 m"

How fast is the rocket moving at 165m?

2. Relevant equations

Ek=mv^2 / 2
Em=eg+ek
Eg=mgh

3. The attempt at a solution

First i found my acceleration.

A= f/m
A= (30- 9.8 x .25) / 0.25
=110.2

Then subbed into

Vf^2 = 0 + 2(110.2)x165
190.1

But my teacher told us to use the law of conservation....

2. Apr 21, 2013

### rock.freak667

Well the work done in moving the rocket 165 m will be equal to the kinetic energy of the rocket at 165 m.

How would you find the work done by the force of 30 N in moving 165 m?

3. Apr 21, 2013

Mutliply 30 by 165 which is 4950

oh.... Then solve for v

4. Apr 21, 2013

### vela

Staff Emeritus
You need to take into account gravity.