Newton's third law and a box push

[mk876]

Homework Statement

I just don't understand how to describe how, if a person pushes on a box and the box exerts an exactly equal but opposite force on the person that the person is still able to move the box...

Homework Equations

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The Attempt at a Solution

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[Quinzio]

What about the feet of that person ?

 

[mk876]

So the feet of the person are meant to absorb the force of the box?

 

[Quinzio]

So what is there beneath the feet of the person ?

Maybe if you drew a very simple drawing of the situation it will help greatly.

 

[mk876]

The earth.

I drew a picture and the forces are the force of the person on the box, the force of the box on the person, the force of the person on the Earth and the force of the Earth on the person (not including the earth/box forces).

So the forces acting on the person are that of the box and that of the earth...

 

[Quinzio]

So the forces acting on the person are that of the box and that of the earth...

Ok, but it's more correct the other way round: the forces acting on the box and on the Earth are the same that those the person exerts.

 

[mk876]

:( I'm confused.

 

[Quinzio]

:( I'm confused.

Don't worry. I think you already got the situation correctly.

 

[mk876]

Ohh so are you saying the Earth will absorb the force of the box and so the person is able to move it?

 

[Quinzio]

Ohh so are you saying the Earth will absorb the force of the box and so the person is able to move it?

Think about a slightly different situation.

The person acts like a spring, pushing with a force F at one end and with the same force F at the other end.
The Earth and the box are then pushed by those Forces.
Since the Earth is very big, it will barely move at all.
But, the Earth is accelerated as well by a very very small quantity.

So you have 2 pairs of forces.
At one end of the spring, the earth-spring (action-reaction), on the other side spring-box (action reaction).

 

[mk876]

Ohhh alright I think I understand a lot better now. Thanks for helping me out :D