How to find energy if mass and height is given?

1. Oct 6, 2011

Echy14

If you are going to throw a 1kg ball vertically upward and reach a maximum height of 18m. A)How much energy is needed?
B)What should be the initial speed of the ball?

My solution:
mgh=1/2mv^2
v=√2gh
v=√2(9.8)(18)
v=18.78m/s

In getting the energy
I derived the work energy theorem
2w=vo^2
2w=18.78^2
w=176.34 joules
I'm not sure if my answer is correct so I would be extremely grateful if someone could check it.

Thanks.

Last edited: Oct 6, 2011
2. Oct 6, 2011

Hootenanny

Staff Emeritus
Correct.
By my reckoning, you are a decimal place off. It is likely a result of your rounding off the velocity.

Note that instead of using the velocity, you could have simply used the fact that work = change in energy = mgh. Do you see?

3. Oct 6, 2011

Echy14

Thanks for the quick reply.

Regarding on the fact that you had just said, that was actually my first solution but seeing that they both have the same result. I decided to use this one instead.