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Ondra

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## 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

*m*2 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.

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