# Newton's Third Law of pushing a box

#### spidey64

action-reactions are in pairs of course, the pusher and the box is one, so then the friction is not part of that but it's own pair with box, is that the idea?

#### kdv

ok, so the box is in dynamic equilibrium...so what's the difference between the force of the box pushing back and the force of friction?
I don't know if this may be part of the confusion but note that the forces forming an action-reaction pair never act on the same object!!! Therefore, they never cancel out! (Unless you consider the two objects as a combined object). For a car, there is a friction force on the car exerted by the pavement and a reaction force on the pavement produced by the car. They act on different objects.

#### awvvu

The box and the ground are action-reaction pairs for the friction force.

Static friction is necessary for us to walk, or for tires to work, but the friction between a moving box and the ground is kinetic friction. You seem to be getting these confused.

#### spidey64

alright, i think i'm getting it now, i gotta go to bed, thanks for your help everyone!

#### Laus102

I am having the same problem, I am taking Into. to Physics and Chemistry and all the other paradigms for the 3rd law make sense to me (e.g. the rocket propulsion, car pushing against the ground etc....), but the example of somone pushing on a box with a force of 100N and the box pushing back on the person with an equal and opposite force always frustrates me. It seems that if I exerted a force of 100N on the box that the force of the box pushing back on me would make the net force in the interaction 0 N. But then how can the box move then, is the box exerting the 100N back on my body and my force of 100 N is still causing the box to move? Does friction play a part in the matter?

#### Doc Al

Mentor
Don't forget that "action" and "reaction" forces (a better term would be "third law pairs") act on different bodies and thus never "cancel out" directly. The force of 100 N on the box describes a force acting on the box. To determine the motion of the box, you need to examine all the forces acting on the box. Similarly, the "reaction" of the box pushing back on you is a force on you.

#### rcgldr

Homework Helper
ok, so the box is in dynamic equilibrium...so what's the difference between the force of the box pushing back and the force of friction?
Zero, assuming the box isn't accelerating. Since static friction is normally greater than dynamic friction, it would have taken more than 100N to get the box to start sliding, and once it was sliding the force could be reduced to equal that of dynamic (sliding) friction, which in this case is 100N, and the box would not accelerate.

Newton 3rd law coexistant force pairs: a forward force applied by the pusher onto the box, and and backwards force applied by the box onto the pusher. A forwards force applied by the box onto the surface it slides on, and a backwards force applied by the surface onto the sliding box. Vertical forces: gravity pulls down on the box and upwards on the earth. The surface the box rests on is compressed: the box applies a downwards force onto the surface, the surface applies an upwards force onto the box, at the bottom side of the surface, the surface applies a downwards force onto the earth, the earth applies an upwards force onto the surface.

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#### Laus102

I just cannot understand how the forces do not cancel out directly yet do in fact cancel out in some way. I understand that the action and reaction forces act on different bodies, but how then do they end up canceling each other out? Do they never cancel out at all? Does the box push back on me until it can no longer match my force and it has to move since my force has offset gravity and friction? Grrrrrrr...

#### Doc Al

Mentor
I just cannot understand how the forces do not cancel out directly yet do in fact cancel out in some way. I understand that the action and reaction forces act on different bodies, but how then do they end up canceling each other out? Do they never cancel out at all?
That's right: Action and reaction forces never cancel out because they don't act on the same body.

If you are able to push on the box with 100N of force, that means the box is also pushing back on you with 100N of force. These two forces don't cancel out.

An example of forces (not action/reaction pairs) canceling would be this: You push on the box with a force of 100N to the right while friction pushes on the box with a force of 100N to the left. Those forces on the box cancel, giving a net force of zero.

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