Why is there no work for someone walking on flat plane

A person is a bad example to use. The human body is a very inefficient machine and it takes work just to keep walking forwards on a flat plane. Replace 'person' with a rolling ball or something and assume no friction.

 

Let's just analyze the case of a single horizontal step (no vertical component) on a horizontal surface. If we can do that, then we can extend it to walking. And we can extend it to an inclined plane.

When you take a single horizontal step, you convert some of the chemical energy stored in your body into mechanical energy. The mechanical energy takes the form of your foot pushing backwards against the ground, the ground pushing forward on your foot, the force on your foot being transmitted to your center of mass through your body parts, and your center of mass moving forward. So the force of the ground causes your center of mass to displace forward, and this translates into work: Fxd = kinetic energy of your body. Now your leading foot sets down on the ground, and your body stops moving. The kinetic energy you had developed is lost, and gets transformed into heat. So the overall effect is chemical energy converting to mechanical work, mechanical work converting to kinetic energy, and kinetic energy converting to heat. Was any work done? Yes, it was done along the way, but it got dissipated. Was there any change in potential energy? No. Was there any permanent change in kinetic energy? No. So, there was no conservative work (i.e., potential energy plus kinetic energy) done? No. But there was mechanical work done.

Does this make sense so far?

Chet

 

That doesn't look correct to me. There's no reason to assume they come to a stop after 10 meters. What happened to the person before or after encountering the inclined plane doesn't matter when it comes to understanding the work done.

 

I don't see what the person is doing before or after the inclined plane has to do with the net force while moving up the plane.

 

What are you talking about? That was my reason.

 

Because of the reasons stated earlier in the thread. Re-read them and if you have questions on what's been said, ask. Don't just throw out an explanation and then demand that we explain why it's wrong. That's a bad way to learn.

 

You've misunderstood. In the example in the other thread you've specifically stated that the person was already at rest and incorporated that into the example. But that needn't be the case if we just want to analyze the work done only when the person is climbing the inclined plane. What happens before or after has no relevance to what happens while on the plane.

 

You have asked your question and it has been answered. As has already been explained, a person walking on a flat surface does have to apply a force to keep moving, and as a result, the person does perform work to keep moving. A simpler example that might be easier to understand involves a smooth ball rolling on a flat, frictionless surface. Since the ball doesn't have to overcome any forces (including friction and gravity), no work is performed.