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 luisa_006 Posts: 1, Reputation: 1 New Member #1 Oct 21, 2007, 03:05 PM
kinetic energy
a 5kg rock is dropped from a height of 92m. What is the kinetic energy when the rock is 40m from the ground?

I know that the formula is kinetic energy=1/2mv(squared)

but I don't have the time so how would I find the speed, v?
 tickle Posts: 23,796, Reputation: 2674 Expert #2 Oct 21, 2007, 03:10 PM
I don't have the time either, you will have to do it yourself in time for your homework assignment tomorrow.
 Capuchin Posts: 5,255, Reputation: 656 Uber Member #3 Oct 21, 2007, 03:26 PM
You can do this in 2 ways, either realise that the loss in potential energy is identical to the gain in kinetic energy, or you can work out the end speed using a SUVAT equation and use the KE equation you have there.

I would say the potential energy way is easier.

Either way, you're going to need to put the work in.
 terryg752 Posts: 197, Reputation: 4 Junior Member #4 Oct 22, 2007, 03:58 AM
Potential energy at height h = mgh

( m = mass, g = force of gravity)

Kinetic energy at height 92 = 0

Hence loss of potential energy in falling = mg (92 - 40) = 52 mg

Hence Kinetic energy = 52 mg

1/2 mv^2 = 52mg

v^2 = 104g

Substitute value g to get the answer!

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