Finding Velocity with Kinetic Energy.

[kencamarador]

Homework Statement

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?

Homework Equations

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

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...

 

[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?

 

[kencamarador]

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?

Mutliply 30 by 165 which is 4950

oh... Then solve for v

 

[vela]

You need to take into account gravity.