BACKGROUND: A few days ago, I came across this question in the book "200 puzzling problems in physics". As the name suggests there are some puzzling problems in it 🙂
The problem I was going through was about a boy trying to move up a height of 20 m using a downward moving escalator. His speed was one and a half times the translational speed of the escalator (relative to escalator). The boy's mass was 50 kg. They asked us to find the work done BY the boy.
So in the solution I was going through,
W done by boy + W done by Normal + work done by weight= 0
the above is the application of Work energy theorem on the boy.
Now my
MAIN QUESTION IS, can the boy do work on himself? I mean can we do work on ourselves to increase our kinetic energy? IN the most basic sense, When we run, we push the ground back and as a reaction, it pushes us right back. Even frictional forces help us walk (by preventing us from slipping). We just push our leg back. So technically the ground does work on us. We don't. Same way for running on the escalator which is pretty dangerous, he pushes the escalator steps down and back which in turn propels him forward. He indeed spends his energy but he technically by the definition of work, he doesn't do any work on himself
Where am I wrong in my argument? Your help will be much appreciated.
Best Answer
Nothing can do work ON itself. Work is done ON an object BY an external force. If an object applies a force to itself (which is by definition an INTERNAL force) then by Newton's Third Law it also applies an equal and opposite force to itself and so the net force that it applies to itself is zero. Hence the work done on an object by its own INTERNAL forces is always zero.
So the only way an object A can change its kinetic energy (or any other aspect of its energy) is to do work on another object B by applying a force to it. Object B then (by Newton's Third Law) applies an equal and opposite force on object A. So the work done BY object B ON object A is equal to the work done BY object A ON object B.