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I'm in a similar scenario as with my other question. I have tried to answer the problem correctly, and I have found similar (and even an identical) problem in my textbook, yet I still can't seem to yield the correct answer with this specific problem.

The example problem in my textbook: A crate is hauled 8m up a ramp under a constant force of 200 N applied at an angle of 25 degrees to the ramp. Find the work done. W = F * D cos(25) = (200)(8)cos(25) = 1450J.

Can anyone give me any advice on how to solve it?

A woman exerts a horizontal force of 4 pounds on a box as she pushes it up a ramp that is 5 feet long and inclined at an angle of 30 degrees above the horizontal.

Find the work done on the box.

I would think that the work would simply be (4)(5)cos(30), but that is not the correct answer.

(I've figured out how to accept answers now, so I'll accept the most helpful answer.)

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Do you have the actual numerical answer? Your method of computation seems ok. I wonder if it is a problem of units. –  Willie Wong Jan 31 '11 at 18:39
consider posting on Physics Exchange also. –  ja72 Jan 31 '11 at 18:44
How about exchanging cos with sin. Does $4 5 \cos(30^\circ)$ yield the correct result? (if the ramp is not inclined at all, no work should be done) –  Fabian Mar 2 '11 at 22:37
First of all dear @Math Student as you say applied force is 4 pounds which (though i seldom work in such units) means that fore applied is 4 * g (acceleration due to gravity). –  HarshCurious Mar 15 '11 at 4:44
In Imperial units, the base unit is pounds as a unit of force, so you should not multiply by g. We have slugs as a unit of mass, and also lbm to confuse things. –  Ross Millikan Mar 15 '11 at 5:09

2 Answers 2

As Fabian said in a comment, the $\cos$ should be $\sin$

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Work done is weight times vertical displacement of the center of gravity. The rest of the problem is playing with the units to get to a consistent set. Well, also friction could add to the work.

Another source of error, is the sin() function, in case it expects a number in radians and you supply it in degrees.

You answer in lbs $\times$ ft is not in Joules. Multiply by 1.35581795 to convert to Joules.

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The answer is actually expected to be ft-lbs. –  Math Student Jan 31 '11 at 19:02

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