Is There a Friction Force on Block 1 in a Three-Mass Pulley System?

[nightflarer]

Block 1 of mass m_1 is placed on block 2 of mass m_2 which is then placed on a table. A string connecting block 2 to a hanging mass M passes over a pulley attached to one end of the table. The mass and friction of the pulley are negligible. The coefficients of friction between blocks 1 and 2 and between block 2 and the tabletop are nonzero and are given in the following table (coefficient between blocks 1 and 2 for static friction is u_s1, for kinetic it's u_k1, for coefficient between block 2 and tabletop, static friction coefficient is u_s2, for kinetic it's u_k2.

Basically, you're supposed to find the acceleration of block 1 and 2, assuming M is large enough that as the hanging block descends, block 1 is slipping on block 2. There are some other questions too.

This was a test question, and I got it right for the most part. One of the questions was whether or not there existed a friction force f_1 exerted on block 1 by block 2, and if so, in what direction. I said there would be one of magnitude m_1*g*u_s1, and that's what a lot of my classmates thought since that's how the teacher had taught us in a problem we did a few weeks ago. However, now she says there is no friction force. What's up?

By the way, does anyone know where I can get access to old AP Physics Mechanic tests, especially the free responses? They'd be a good help for me for preparing for tests in the future. Thanks a lot.

 

[Doc Al]

One of the questions was whether or not there existed a friction force f_1 exerted on block 1 by block 2, and if so, in what direction. I said there would be one of magnitude m_1*g*u_s1, and that's what a lot of my classmates thought since that's how the teacher had taught us in a problem we did a few weeks ago. However, now she says there is no friction force. What's up?

Assuming your description of the problem is accurate, of course there's a friction force between the two blocks. But since you earlier said that block 1 is slipping on block 2, why did you use the coefficient of static friction?

 

[nightflarer]

Doc, the part I was referring to was a different section where M was small enough so that the blocks remain at rest when released. Sorry if that was unclear.

 

[Doc Al]

Big difference. If the blocks don't move there will be no friction force between them. (Just like if you rest a book on a horizontal surface: What's the friction force on it? None. If you try to push it and it resists, then there's a friction force involved.)