# Energy conservation Law

1. Jan 21, 2008

### couris

Hello,
http://fm92-1.com/physics.jpg" [Broken]
The initial speed is 0 m/s
We let go of the rope
What is the system's kinetic energy when the little block is at 15 cm from the ground?

What's wrong with my work?
$$E_{t}=E_{P}+E_{F}+E_{K}\Rightarrow E_{K}=E_{t}-E_{P}-E_{F}$$
where Et is total energy, Ep is potential energy, Ef is friction energy and Ek is kinetic energy.

$$E_{K}=0.75*9.81*0.45-0.75*9.81*0.15-0.1*2.5*9.81=-2.4$$
The answer is 1.47, but i get -2.4
What to do?
Thank you

Last edited by a moderator: May 3, 2017
2. Jan 21, 2008

### Poop-Loops

Your last term, energy lost due to friction, is calculated wrong. Friction energy is force times distance. What you have is force. You need to multiply it by a distance (how far the block moves) to get energy.

I don't know if you get the right answer, but that's one thing that's wrong that I could find.

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