Understanding Newton's Third Law: Forces on a Box

[Peter G.]

I'm a bit confused: Let's take a box standing on a table:

So there are two pairs of force acting on it?

Can anyone help me with the forces on the box?

Thanks,
Peter G.

 

[Delphi51]

Gravity pulls it down. What force holds it in place?

 

[issacnewton]

There is a force of gravity acting downwards. Can you think of the reaction to
this force ? Remember real forces always come in action-reaction pair.
What other forces can you think of, which are acting on the box ?
If force of gravity is acting downwards, what keeps it from falling down ?

 

[Peter G.]

The table exerts an equal and opposite force on the box?

But look what confuses me: "Be careful where two forces are equal and opposite but have nothing to do with the Third Law. For example, a block, mass 3kg resting on a horizontal table has two forces acting on it. Its weight, 30 N and the reaction from the table that is also 30N. These two forces are opposite and equal but they are acting on the same body and so have nothing to do with Newton's Third Law (?). We have seen in the last bullet point above the force that pairs with the weight of the block. The one that pairs with the reaction force is a downward force on the table."

It says in the previous bullet point that the Earth pulls down on the box and therefore, the box pulls up on the earth.

 

[Delphi51]

Sorry, I guess I gave a wisecrack answer without thinking it through!
For 3rd law we must think of "A pushes on B and B pushes back on A"
So A is the block, B is the table. The block presses down on the table, and the table pushes back up on the block with an equal force. This would be true even if the whole system of block and table were accelerating up or down.

 

[issacnewton]

Peter, remember that the action-reaction pair in the third law ALWAYS act on different
bodies. This is a HUGE point of confusion for the students while learning this law. So two
important things. First, the forces in nature ALWAYS come in action-reaction pair. While
walking on a road or while in a bus, you can look at different bodies around you and think
about some force and try to come up with the reaction to that force.
Second this is that these action-reaction forces ALWAYS act on different bodies, not the
same body. So, while solving your next problem involving third law, slap yourself first
(kidding) and remember these two points. You will NEVER make mistakes, believe me.

You will be surprised to know that this third law was a huge source of confusion for
Newton himself.

 

[Peter G.]

Thanks a lot for the help guys, but for me to feel comfortable I have to comprehend these two things:

When the box is standing on the table, its weight exerts a force on it and the table exerts a force on the box. How are these two forces acting on the same body? Isn't the weight acting on the table and the reaction force acting on the box?

My teacher said that there are four forces involved: I know two: The Earth pulls down on the box therefore, there is a reaction force, we pull up on the Earth. But what are the other two?

Sorry, I hope I got my point across...

Thanks once again,
Peter G.

 

[Tim D.]

Force of Gravity (Weight) and the Normal Force. if its just sitting at rest. The "Weight" would be due to the Earth and the "Normal" would be due to the table. along the y-axis they need to cancel out.

 

[Delphi51]

The Earth pulls down on the box & the box pulls up on the Earth.
The box pushes down on the table & the table pushes up on the box.