Question

A 150 N boy rides a 60 N bicycle a total of 200 m at a constant speed. The frictional force against the forward motion of the bicycle equals 35 N. How much work does the boy do? Explain your answer.

Answer 1

I might be looking at this wrong but wouldn't the Work done by the boy = 0. This is because he pedals up and down which is perpendicular to the direction of motion. Again I might be over thinking this problem. Anyone else?

Answer 2

Work Done is the force applied over a specified distance i.e. \[WD=F\times d\] In this case the work done is against friction. Hence the answer is \[WD=35\times200=7000\rm{J}\]. Note that because you are moving at a constant speed, the weight of the bike and the rider are not factored in, as you are not accelrating them. the only acceleration comes from matching the deceleration due to friction, so that the net acceleration is zero. Also, the weights of the bike and rider are the gravitational weight, and are at right angles to the motion. If you had to raise them in height (that is go up a hill) you would then have to do work agains the gravitational field.