Why Is Lifting a Box Considered Work in Physics?

[russ_watters]

My question is that as I walk, friction is the force that helps me in moving forward. So, I can say that friction causes the displacement of my body. So, if force is acting on my body, force is acting on the box as my body and box are the same system.

Fair enough: my answer applied equally to you and the box: The forces/works sum to zero. If you are under the impression that walking forward involves only forces to push you forward, you are mistaken. Every step except the first and last starts with a push forward and ends with an equal push backwards.

 

[Drakkith]

My question is that as I walk, friction is the force that helps me in moving forward. So, I can say that friction causes the displacement of my body. So, if force is acting on my body, force is acting on the box as my body and box are the same system.

This is not accurate. Friction is the reason you can accelerate forward, it is not the reason you continue to move forward. Once moving, nothing is required to keep you moving. When walking at a steady pace, friction keeps your feet from slipping out from under you and prevents you from falling to the ground. Ever seen someone slip on ice? They continue to move forward even as they fall.

The work being done here is not to keep you and the box moving forward, but to extend and contract your leg muscles to keep your feet moving and prevent you and the box from falling to the ground, where friction would stop both of you from moving forward. This is easy to see when you consider what would happen if you simply let go of the box and stopped walking. The box continues to move forward at the same velocity even as it falls to the ground.

 

[Dale]

So, if force is acting on my body, force is acting on the box as my body and box are the same system.

You cannot do an analysis this way. If the system you are analyzing is the box+you then all you can say is that the friction force is acting on the system. You cannot consider the system of the box+you and then make any conclusions about just the box.

If you want to make conclusions about just the box then you MUST analyze a system consisting of just the box.

 

[sophiecentaur]

Okay. Now, I get it. It was a moment for me when I made the silliest of mistakes. Sorry of all the trouble especially to @sophiecentaur who got a bit angry.

HAHA.
Only a little bit!
But we managed to weather the storm, I think.

Looking back, if you want to consider what is going on inside the you / box system and to look at all the forces at work there, you would, in the limit, be looking at every single part of you (bones, muscles) and the box. This would be needlessly complicated and would still not be giving a better answer about work done on the box. 'We' do it the way 'we' do it because it works and can take us further when we want it to. Pretty much all Science works that way - which is not necessarily intuitive.