Physics

When a block slides a certain distance down an incline the work done by gravity is 300 J. What is the work done by gravity if this block slides the same distance up the incline?
a) 300 J
b) zero
c) -300 J
d) It cannot be determined without knowing the distance the block travels.
e) It cannot be determined without knowing the coefficient of friction

So would that make the answer c) -300 J? Work= Fd and so the force is the same but the direction is different.



No, that is not the answer. Of the 300J, how much was lost to friction? Without that information, ....

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  1. First, you should have appreciated that there is no such thing as negative work (you can't "undo" work by doing work in a different direction). This immediately eliminates answers b and c.
    The remaining answers rely on your clear understanding of the definition/equation for work, namely W=F*d. Specifically you need to understand that the force must be parallel to the distance traveled. Now think about it; when the block is slid down the ramp, sufficient force is required to overcome any friction that is present. However that same friction is present on the way back up. The frictional force resisting motion is a function only of the geometry of the ramp, and the materials of the ramp and block. Whether going up or down, the force required to overcome friction is identical. This eliminates answer e. So we're left with answers a and d.
    Remembering that the force involved in the calculation of work is parallel to the motion consider again the force required to move the block down the ramp. In this scenario the force down the ramp is supplemented by the gravitational pull of its mass. It should now be clear that this force will be less than that required to move the block up the ramp. Conversely, the force required to move the block back up the ramp must be greater than that required on the way down. Since W=F*d and the F's are different on the way up than on the way down, there is no way to be sure that the work will be identical.
    The answer is therefore d. Only by knowing the distance traveled with the higher force going up the ramp, can the answer be determined.

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  2. Your response is incorrect.

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  3. The answer is c because the gravitational potential energy is a function of position only, so the work done by the gravity for the block sliding the same distance up the incline is−300 J.

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