Homework Help: Given Eki + Egi = Ekf + Egi

1. Nov 1, 2005

runicle

Given the equation Eki + Egi = Ekf + Egi
Which is initial kinetic energy plus initial potential energy equals final kinetic energy plus final potential energy.
If i were to measure the speed of a rollercoaster at the end of its position of a height of 3 and it's initial height is 12 which of these answers are true?
initial / final
1/2*m*vf2 + mgh = 1/2*m*vf2 + mgh
1/2*0 + 9.8m/s2 (12m) = 1/2*vf2 + 9.8m/s2(3)
initial / final
1/2*m*vf2 + mgh = 1/2*m*vf2 + mgh
1/2*0 + 9.8m/s2 (9m) = 1/2*vf2 + 9.8m/s2(0)
*note mass is not included due to they cancel out.

2. Nov 1, 2005

daniel_i_l

They are both right. It depends on were you take you referance point - at 0 like the first case or at 3 like the second. In both cases you should get the same answer.

3. Nov 1, 2005

Andrew Mason

You have to tell us what the speed of the rollercoaster is when it is at the top.

Also, your suggested answers do not make sense. The first and third are true by definition. In the second and fourth, what do the (3) and (0) refer to?

AM

4. Nov 1, 2005

daniel_i_l

I thought that he meant that at the top it was at rest, 0 and 3 are hights

5. Nov 1, 2005

runicle

what was i trying to find?

6. Nov 1, 2005

daniel_i_l

the final speed?

7. Nov 3, 2005

runicle

yes

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Boo yeh!!