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Old Oct 29th 2008, 09:11 PM   #1
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Work and Energy

Question
Rene, whose mass is 85 kg, skis down a hill, passing a point Z with a kinetic energy of 9700J.

If friction is ignored, to what maximum height, h, can Rene ski?

Relevant Equations
ΔEp = -ΔEk
-ΔEp = ΔEk

for relevant equations I am not quite sure because one teacher showed us to use the one above while another teacher showed us the one below.

Attempt
For this question I am assuming that you use the above one because you are trying to find the final height "hf" for this question. So what I did was:

-ΔEp = ΔEk
-(mghf - mghi) = (1/2)mvf(squared) - (1/2)mvi(squared)

I continued to plug in the numbers and isolate the final height "hf", but then I encountered a problem because I don't know what the final velocity is "vf" so I don't know how to proceed.
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Old Oct 31st 2008, 07:51 PM   #2
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Originally Posted by Inertialforce View Post
Question
Rene, whose mass is 85 kg, skis down a hill, passing a point Z with a kinetic energy of 9700J.

If friction is ignored, to what maximum height, h, can Rene ski?

Relevant Equations
ΔEp = -ΔEk
-ΔEp = ΔEk

for relevant equations I am not quite sure because one teacher showed us to use the one above while another teacher showed us the one below.

Attempt
For this question I am assuming that you use the above one because you are trying to find the final height "hf" for this question. So what I did was:

-ΔEp = ΔEk
-(mghf - mghi) = (1/2)mvf(squared) - (1/2)mvi(squared)

I continued to plug in the numbers and isolate the final height "hf", but then I encountered a problem because I don't know what the final velocity is "vf" so I don't know how to proceed.
Using
$\displaystyle 9700= mv^2/2$
then you can get the velocity at that time

-ΔEp = ΔEk
$\displaystyle -mgh=mv^2/2$
$\displaystyle h=-v^2/2g$
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