Energy/Work Q: Find Speed of Block in Frictionless & 0.350 Coeff Surfaces

[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 high school 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 don't understand is that my teacher said KE + PE +PEs =Wf (which is work of friction) but in high school (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

 

[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.

 

[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.