So I have a question:
In a children's park, there is a slide which has a total length of 10 m and a height of 8m. Vertical ladder is provided to reach the top. A boy weighing 200 N climbs up the ladder to the top of the slide and slides down to the ground. The average friction offered by the slide is three tenth of his weight.
Find the work done by the ladder on the boy as he goes up. Neglect any work done by forces inside the body of the boy.
My question is: how does the ladder do any work on the boy?
So the boy pushes on the ladder with his feet at a point and the ladder pushes back at his feet at that point.
But don't point forces do no work (in the words of my teacher) or something?
Best Answer
You are correct. In the rest frame of the ladder, the point of contact between the ladder and the boy's foot never moves, so the ladder does no work. It is precisely the work done by forces inside the body of the boy that would be doing the work here. The fact that the problem says to ignore these forces might indicate they indeed want you to say the answer is $0\,\rm J$
However, physics problems like these can be written sloppily; even ignoring the issue raised by the OP, if the author of the problem is assuming the ladder does a non-zero amount of work, then they have not provided enough information, as you do not know the velocity of the boy at the bottom and at the top of the ladder. In this case, it is likely that whoever made this problem was assuming the boy starts and stops at rest when going up the ladder, and was really asking how much work is needed in order to get the boy to the top of the ladder; then the simple answer is just $mgh=1600\, \rm J$, of course.
In this particular case I would recommend just asking your teacher to clarify.