Homework Help: Friction - conceptually confused!

1. Jun 7, 2012

sideman123

1. The problem statement, all variables and given/known data

a 2 kg block rests on a horizontal frictionless surface, and is initially at rest. a 1 kg block rests on top of the 2kg and initially moving at v=3m/s (to the right). the coefficients of friction between he two blocks are MUs=.2 and MUk=.1 . A force is applied to the top block such as to keep it always moving at the same velocity. you can assume the top block never falls off the bottom block. what will the bottom block do?

the answer is always move at a constant speed to the right.

i dont understand this. i know friction exists between the top and bottom block, but how does that get the bottom block to move with the top block to the right. I generally believed friction acts in the opposite direction of which the force is applied?

if it would help, i can post a picture

thanks

2. Jun 7, 2012

Staff: Mentor

I guess I don't understand the problem. Are the two blocks moving together? Is the top block sliding along the bottom block? Why is a force needed to keep the blocks moving on a frictionless surface?

Sure, post a picture.

3. Jun 7, 2012

sideman123

see the pics-

thanks

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4. Jun 7, 2012

Staff: Mentor

Friction acts to prevent or oppose slipping between surfaces. If the top block is pushed to the right, the friction from the bottom block will push to the left. And if the bottom block exerts a friction force on the top block, then the top block will exert an equal and opposite friction force on the bottom block--to the right, in this case.

That said, I still don't understand the problem.

5. Jun 7, 2012

ehild

A free-body diagram always helps...
The external force and friction from the bottom block act on the top block. The top block does not accelerate, so the kinetic friction is balanced by the external force.
Only friction from the top block acts on the bottom block. It accelerates till it reaches the speed of the top block when the friction becomes static.

ehild

(Edited)

Last edited: Jun 7, 2012
6. Jun 7, 2012

azizlwl

It is just a relative motion.
To the block below, there's a motion to the left relative to the above block(to the right).
Thus static frictional force is in direction of the top block. Not in direction as you assume.
The static friction will balance this until acceleration greater than μsg.

Last edited: Jun 7, 2012