# Efficence - Calculate the Force needed to life this load

1. Mar 18, 2012

### I-need-help

1. The problem statement, all variables and given/known data
Consider a mechanical car jack. To change a wheel it may need to lift a load of 2000N. Suppose that one rotation of the screw raises the car by 10mm. You apply a force to the end of the lever, 0.5m long, through a full circle. The jack is only 40% efficient. Calculate the force that you have to apply.

(This is a textbook question from OCR, AS Physics.)

2. Relevant equations

probably to do with moments:
anti-clockwise moments = clockwise moments

work done:
work done = force x perpendicular distance...

GPE:
gravitational pe = mass x g x height

I don't know where to begin, etc..

3. The attempt at a solution

There is answer at the back of the textbook, but I won't show it for now, so that it doesn't affect your way of workings, or anything... I'll show it if needed though.

Thank you for any help!

2. Mar 18, 2012

### LawrenceC

The work done by the movement of the lever must equal the work done in raising the vehicle. With the jack not being 100% efficient due to friction, you'll have to supply 40% more work to raise it.

So begin thinking about work being done.