Friction Problem Homework: Draw Forces, Determine Accelerations

[emant777]

Homework Statement

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Consider three versions of a problem with three blocks of masses m, 4m, and 3m. The small mass is always on top of the large mass, and the 3m mass is accelerating downwards (but at different rates). There is friction between the m and 4m block but not anywhere else. The coefficients of kinetic and static friction are 0.6 and 0.9 respectively. Pulleys and strings are massless.

a) Draw the forces of friction in each case. Which are static and which are kinetic?

b) In which case is the acceleration the greatest?... the least? Justify your choice.

c) Determine the accelerations in each case and check your answer to (b).

Homework Equations

sum of F = ma.
Fr = uN

The Attempt at a Solution

So in each of the cases the friction must be kinetic?

Please help.

 

[G01]

I don't see how any of the friction forces can be static.

You said that the friction only exists between bottom and top blocks. If this is the case, is the top block moving with respect to the bottom block? What does this tell you about the type of friction?

 

[SRode]

Those diagrams are poorly drawn, but from what it looks like you need to compare the constraints between the various scenarios. In one problem you have only static friction and in the other only konetic. Examine what acting forces are different and you will see which frictional coefficient to use.