Two objects attached by pulleys

1. Jul 4, 2010

sona1177

1. The problem statement, all variables and given/known data
There are two objects attached by a pulled on a cline. please click the link to see the diagram.

http://session.masteringphysics.com/problemAsset/1010976/24/MLD_2l_2_v2_2_a.jpg

if m2 weighed 2 kg and m1 weight 6 kg, which direction does the static friction point for m2? Does it point up and to the left or down and to the right. I said down and to the right because without friction, the pulley would pull it up, so since static friction points in the direction the object would move if there were no friction, then it would be down and to the right. I am so confused. someone help. I can figure the whole problem i have here but i just don't know which direction the static friction points which messes up my freebody diagram, which in turn messes up the signs in the equations, which in turn messes up the answer. Please help!

2. Relevant equations

3. The attempt at a solution

2. Jul 4, 2010

hikaru1221

Your reasoning is okay. What if the static friction points up and to the left?

3. Jul 4, 2010

sona1177

Then wouldn't the object just move? But is my answer correct, that the object moves down and to the right? Please don't mess with my mind! :) This concept was messing with my head for 3 hours before i chose to come here.

4. Jul 4, 2010

hikaru1221

If the friction "helps" the object move, is it "friction" anymore?
You're correct. I just ask some more to bolster your confidence. Hope this help

5. Jul 4, 2010

sona1177

Thank you! Right, I said the object would just move if static friction acted up and to the left. so its not friction anymore :) Thanks again.

6. Jul 4, 2010

hikaru1221

Uhm... I'm not sure if you got what I meant or not. Okay, make it a bit harder
What if m1=2kg and m2=6kg?

7. Jul 4, 2010

sona1177

In this case, m2=6 and m1=2, static friction points up and to the left because without friction, the pulley would pull it down and to the right.

8. Jul 4, 2010

hikaru1221

You correctly pointed out one case but you missed a case . In the extreme scenario when the plane is horizontal, m2 is pulled to the left and thus, friction points to the right, correct? The angle between the plane and the horizontal direction must be taken into account. If m2=2.001kg which is very close to m1, you can get the image of this situation, can't you?

The main point I'm trying to point out is that when m2 seems to move to the left, friction acts to the right, and when m2 seems to move to the right, friction acts to the left. That's why I asked "If the friction "helps" the object move, is it "friction" anymore?". When the mass tends to move in some direction, friction always acts in the opposite direction. This might be helpful to you in more complicated problems. But if you find it totally a mess, then forget it; you may come up with it some day

Last edited: Jul 4, 2010