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Conservation of mechanical energy of ball of mass

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12
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1. Homework Statement
A ball of mass 240 g is moving through the air at 20.0m/s with a gravitational potential energy of 70J. With what speed will the ball hit the ground?

2. Homework Equations
Eg = mgh
Ek = 1/2mv^2
W = mgd

3. The Attempt at a Solution
this is what i did:
at 0m, potential energy is 0 so kinetic energy must have 70J now.

Ek = 1/2mv^2
70 J = 1/2(0.24kg)v^2
v = 24.15 m/s

I dont know what to do with the 20 m/s that was given and yeah....please help!
 

Answers and Replies

458
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The potential and kinetic energy that it had initially is now all kinetic by the time it reaches the ground.
 
454
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you have

potential energy + kinetic energy = total energy = constant

What is the total energy whe the ball is moving through the air? it's not 70 J.

At a height of 0 m, potential energy is indeed 0, so all the energy must be kinetic.
 
5
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since he total mechanical energy is conserved equate the initial total mechanical energy
(pe+ke)to the final toal mechanical enrgy.
remeber pe=mgh ,so what is pe at ground level?
 
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