# Energy/work question

1. Mar 13, 2015

### jared bernstein

Hey guys I am in intro physics in college and took physics in high school. I have a general question about work/energy. In highschool we used work= change in total energy and total energy = PE +KE +Q (Internal) which =work. In class there was a problem that read
2.00 kg block is attached to a spring of force constant 500 N/m. The block is pulled 4.50 cm to the right of equilibrium and released from rest.

(a) Find the speed of the block as it passes through equilibrium if the horizontal surface is frictionless.

(b) Find the speed of the block as it passes through equilibrium (for the first time) if the coefficient of friction between block and surface is 0.350

What I dont understand is that my teacher said KE + PE +PEs =Wf (which is work of friction) but in highschool (the equation above the friction or Q is on the other side and she says that the friction is equal to W.

Can anyone explain!!

Or simply do the problem out for me I am getting mixed messages!

thanks

first post btw

2. Mar 13, 2015

### Rosenti 87

At question b) is it right equation?
Maybe we can ignore PE for horizontal spring. We can write: KE+PEs+Wf= konstan.

@ is it your theory right? I Think: work= the change of KE or work= the change of PE. Sorry if I am wrong.

3. Mar 13, 2015

### Rosenti 87

At question a). The problem said frictionless. I think, in the equilibrium, the KE =total energy. And at the initial position (pullet 4.50cm) the PE (from the spring)= total energy. So, we get: 1/2 m v² = 1/2 k x².