# Homework Help: Dynamics pulley system.

1. Mar 29, 2010

### weedannycool

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

A car mechanic wants to lift the engine from a car using a rope and pulley
arrangement. He considers two options, shown in Figure Q1a and Figure Q1b. The car engine
has a mass, m, of 200kg, and the maximum force that the mechanic can exert on the rope, P, is
1500N. Use a free-body diagram approach to derive expressions for the acceleration
experienced by the car engine in each case. Use these expressions to calculate the vertical
acceleration of the engine if the mechanic exerts the maximum force on the rope. You may
neglect the mass of the pulley systems, and assume that they are frictionless.

Why can the man lift the engine in case b)(right of the figure). but not a)(left of figure)?

2. Relevant equations

$$\sum= Fy$$
$$\sum=Fx$$

3. The attempt at a solution

this is a dynamics question. i think wat i would do is find the sum of forces in the X and Y direction. however i am not sure. either the or it would be using circular accelertion.

Thanks

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Last edited: Mar 29, 2010
2. Mar 29, 2010

### PhanthomJay

3. Mar 29, 2010

### weedannycool

soz for the rubbish drawing

4. Mar 29, 2010

### PhanthomJay

That is mechanical advantage working for the mechanic. Draw a free body diagram of the engine in case a; then draw a free body diagram of the engine and pulley in case b. Use newton's laws. Acceleration of the engine is not 'circular', it is 'linear' (it moves and accelerates straight up, if it is to move at all). Note that tensions in the rope on either side of an ideal pulley are the same.