Homework Help: Friction/KE problem

1. Oct 12, 2009

1. Two blocks, A and B (with mass 50 kg and 100 kg, respectively), are connected by a string, as shown in the figure below. The pulley is frictionless and of negligible mass. The coefficient of kinetic friction between block A and the incline is µk = 0.21. Determine the change in the kinetic energy of block A as it moves from C to D, a distance of 25 m up the incline if the system starts from rest.

http://img26.imageshack.us/img26/5803/p564.gif [Broken]

I used
a1=-a2
T-m1g*sin37=m1a1
T-m2g=m2a2 -T+m2g=m2a1
m2g-m2a1-m1g*sin37=m1a1

3.

Solved for a (1.142 m/s^2), solved for Vf (.7556 m/s) and used the equation KE=.5mv^2. I did something wrong, but I'm not sure what. Please help

Last edited by a moderator: May 4, 2017
2. Oct 12, 2009

rl.bhat

In your attempt you have not taken into account the frictional force, which acts in the opposite direction of motion of A

3. Oct 12, 2009

So should i use T-m1g*sin37-ukn=m1a1?

4. Oct 12, 2009

rl.bhat

Yes.
What is the normal force?

5. Oct 13, 2009

Ummm, should be mg*cos37 right?

6. Oct 13, 2009

Jebus_Chris

You can also solve the problem by using energy.

7. Oct 13, 2009

rl.bhat

Right.

8. Oct 13, 2009