Friction on Aluminum & Copper Blocks: Find Sum of Magnitudes

[Ondra]

Homework Statement

A 2.00-kg aluminum block and a 6.00-kg copper block are connected by a light string over a frictionless pulley. They sit on a steel surface as shown in Figure P5.64, where theta= 30.0°. When they are released from rest, will they start to move? If so, determine (a) their acceleration and (b) the tension in the string. If not, determine the sum of the magnitudes of the forces of friction acting on the blocks.

https://drive.google.com/file/d/0B4ksjSfetGUOeUdXX0tKODRWVjg/view?usp=sharing ---- IMAGE

Homework Equations

1) I don't know how to calculate the last part: "If not, determine the sum of the magnitudes of the forces of friction acting on the blocks."

2) Is it so far correctly calculated?

The Attempt at a Solution

For the system to start to move when released, the force tending to move m2 down the incline, m g 2 sinθ, must exceed the maximum friction force which can retard the motion:

F = f(steel) + f(copper)

= µ1m1g + µ2m2g

= 0.61 x 2.0 x 9.8 + 0.53 x 6.0 cos(30) x 9.8 = 38.9 N

Force tending to cause the system to move = 6.0 x 9.8 x sin(30) = 29.4 N
Hence it will not start to move when released.

 

[Ondra]

It just hit me... Did I already solved the: "If not, determine the sum of the magnitudes of the forces of friction acting on the blocks." Part? :D

 

[haruspex]

µ2m2g

That is not correct for the block on the slope, but I see you used the right version in the next line.

Did I already solved the: "If not, determine the sum of the magnitudes of the forces of friction acting on the blocks." Part?

Possibly. You calculated two forces. Which one are you suggesting as answer to part b?
Before answering, what exactly is the standard equation relating to static friction?

 

[Logical Dog]

The formula for friction is F = coeffiicent of friction * Normal contact force. For M2, the normal contact force is not the weight. but you have corrected it lol. Where did you get the coefficients from? Online? I see its 0.61 between aluminium and steel.

Also, I know I posted in your earlier thread that got deleted, so please try to check out these two websites:

scroll down for mechanics:
http://www.cimt.org.uk/projects/mepres/alevel/alevel.htm

 

[haruspex]

The formula for friction is F = coeffiicent of friction * Normal contact force.

Not quite, and that is important here.