Newton's third law is confusing. Help me?

Question

Every action has an equal and opposite reaction; therefore, if I push a pencil across the table, it will push back with the same force and they will cancel out resulting in the pencil not moving at all, but it does... I'm so confused by what Newton's third law actually means. Can you enlighten me?

Answer 1

Every action has an equal and opposite reaction;



What is action . In Physics there is a separate quantity called action which is energy *time .



The above statement is not stated in its correct form That is the reason every one finds it difficult to understand third law.



The correct statement is if A applies a force on B, then B will apply equal and opposite force .



Thus if you find a force , correspondingly there will be another equal force on some other object.



Forces occurs in pairs and not in single .

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The important point to note is that



The Newton's third law says about forces on two different objects .



To find the motion of an object , we must consider the forces acting on the single object and apply newton's second law of motion.



In your case the pencil moves because the net force action on it is not zero.



You are not moving because the net force action on you is zero .



If there were no firction, you too will move back if you push a pencil.

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Answer 2

This is a question many people ask. The simple reason why is because your finger has more resistance to movement (from mass, and also friction).



And they do not cancel out because the two opposite forces aren't directed at the same object. A force (singular) is acting on each object that are pushing against each other. So "A FORCE" is acting on the finger and "A FORCE" is acting on the pencil. They don't cancel out because the opposite forces are being directed on two different objects.









Now to explain why the pencil moves...

The Newtons (N) is the unit of force. The equation is "Force = mass * acceleration"

Let's say you push the pencil with your finger with 2 Newtons.

Now listen carefully...



A force of 2 Newtons is acting on the pencil and will move.

A force of 2 Newtons is ALSO acting on your finger.

The reason why your body doesn't move while the pencil moves is because of the mass and any relevant friction. This is based on Force = mass * acceleration.



The pencil has a small mass so will experience some acceleration (velocity is increasing from a velocity of zero). Basically 2 Newtons = (small mass)*(relatively large acceleration).

And since the pencil is rolling friction isn't really relevant.



Your body (that has your finger) has a very large mass in comparison. So in a frictionless and ideal environment, your body (finger) will experience very SMALL acceleration because you have a large mass. Remember that both the pencil and finger receives the same force. So 2 Newtons = (large mass)* (relatively small acceleration).

So your body will experience a very SMALL acceleration in an ideal environment.



In reality there is friction (which acts opposite of any force). Basically if there is a force pushing you forward, then any friction will pull you backwards (this is why when you push a box along a surface, it slows down since friction moves opposite of the force).

In the case of reality, there is a LOT of friction that can exist between your shoes and the ground (or you and the chair). The reason why you don't move at all in reality is that the force of friction would be much greater than the force of 2 Newtons that comes from pushing the pencil.

(and if you want to know more, you only accelerate if you overcome the static friction that exists, which is the friction that exists when you are at rest. In the case of pushing a pencil pencil, the static friction is greater than 2 Newtons, so you don't move at all).



This is the most thorough explanation I can give. Hope it helps.

Answer 3

Remember that Newton's Third Law is about the interaction forces between two objects, *not* about the forces on one object.



So, when you push on the pencil, it does, as you point out, push back with an equal and opposite force. But be careful here! This equal and opposite force is not ON the pencil! It a a force FROM the pencil, on YOU. That's what Newton's Third Law is about. You push on the pencil, and the pencil pushes on you. Those two forces can't "cancel", because they're not on the same object. One force is on the pencil, and one is on you (which I guess I've said three times now, but that's OK, because this is a concept that is both important, and often misunderstood).



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@ L. E. Gant: You may want to reconsider your claim that when you push a pencil across a table, the movement of your finger is so small as to be un-noticeable.

Answer 4

Ok, so lets think about another scenario. Somebody grabs an egg and throws it at a wall with lets say 4N. The egg obviously would break, but the only way it could break is if a force is being applied on it in the opposite direction. If only one force was applied to it, then it would continue to go in one direction, but as the wall also applies a force of 4 Newtons, the egg breaks. With your pencil scenario, the pencil travels because of a force of lets say 2 N, in order for the pencil to stop, a force must be applied with the same equal force of 2N, otherwise it would continue moving, if a larger force of 3 N is applied in the opposite direction, then the pencil would reverse backwards. With what you said, how the pencil doesn't travel anywhere, it does obviously, a force is applied onto the pencil, and then an equal force is applied in the opposite direction to make become completley stationary.

Hope I helped.

Answer 5

No. They don't cancel out quite that simply.

Basically, F = ma (force = mass * acceleration)

So, the pencil accelerates. You (and your finger) feel the same force (the reaction), but your mass is extremely large compared to that of the pencil, so the acceleration (and hence movement of you and your finger) is so small it's unnoticeable.

If you were pushing against a huge rock, not able to move it, the rock is exerting the same push against you, but you and the rock don't move...

Answer 6

Sure, if I jump, I fall and hit earth, Earth moves 0.0000000000000007 m or something like that, force is proportional to acceleration, F=ma, so as force transfers between objects the total kinetic force (and basically the acceleration it causes) dissipates in either direction, so negative and positive acceleration, one slows down, the other speeds up, the forces don't actually cancel in most cases. If one object collides with another object of same mass in a vacuum, each with the same speed, they will either bounce back with the same speeds reversed for each object, slightly smaller speeds for each object, or stick together, depending on what kind of collision it it: Perfectly elastic, elastic, inelastic, or perfectly inelastic. A catcher stops a ball: inelastic. There are virtually no perfectly elastic or inelastic collisions in nature. An inelastic collision, however, is a great example of "canceling" the forces, but only dissipating the remaining energy as heat or something else.