Physics on Potential and Kinetic energy?
A stone with a mass of 0.1 kg is drop from a height of 80 m.
Calculate PE and KE
a)the time it was drop?
b)after it travels 20 m
c) at an instant it touches the ground
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so i've already solved a) and b), how do i solve for c)?
a) i got KE=0, PE= 78.4 J
b) after 20 m, i got PE= 58.8 J and KE=19.6 J
so please help me solve c), what is the solution? please show the whole process. thanks a lot
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Verified answer
c)
PE = mgh
When the stone touches the ground it have no height, So: h=0
PE = 0.1 x 9.8 x 0
PE = 0
When the stone is at its maximum height from the ground, all of the energy in the stone is potential energy. When the stone falls to the ground that potential energy is converted to kinetic energy So the potential energy before the stone dropped is equal to the kinetic energy just before the stone hits the ground. So we get this:
KE just before the stone hits the ground = PE before the stone is dropped
So KE is equal to the amount of PE you calculated in (a) *before the stone is dropped*. That is 78.4 J.
Use conservation of mechanical energy:
PE @ 80 m = KE @ 0 m + PE @ 0 m
mgh = 1/2 mv^2 + mg(0) = 1/2 mv^2
So PE@80 m = KE@ 0m = 78.4 J
Total energy remains constant.
at the instant it touches ground, h=0, so using PE=mgh, PE=0
so kE=total energy -PE=78.4J