Sometimes a system's internal chemical energy is converted into mechanical energy and thermal energy with no work being done on the system by external forces. For example, at the beginning of this section we described the energy conversions that take place when you start running. In order to move forward, you push back on the floor and the floor pushes on you with a static frictional force. This force causes you to accelerate, but it does not do work. It does no work because the displacement of the point of application of the force is zero (assuming your shoes do not slip on the floor). Because no work is done, no energy is being transferred from the floor to your body. The kinetic energy increase of your body comes from the conversion of chemical energy derived from the food you eat.
So when we walk on the ground, friction does no work, only providing grip so as our chemical energy can become kinetic energy.
But let's say I walk forward/accelerate on a moving train, with the train moving at a constant speed. And there is an observer sitting watching this outside. They would see friction doing positive work on me relative to the ground?
The train is moving at constant velocity. The energy the person gains comes from internal energy, like chemical potential energy. So why does it look as if friction from the train to the person is doing work. I'm confused, what am I seeing. This makes no sense to me.
Edit/picture is to end question about "does friction do work walking". This questions is not about that. I am trying to understand how in one frame, lets say walking, friction does no work, but in another frame, it appears it does. Maybe I am just confusing reference frames... not sure