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Newton's third law states that to every action, there is an equal and opposite reaction.

If that's the case, then how do things move at all? Shouldn't all applied forces be canceled by the equal and opposite force it experiences from the object they were applied to?

Brandon Enright
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ThisIsNotAnId
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    Simple, the things move when they feel a force exerted upon them. They don't care if the other object feels a reaction. – Ana S. H. May 04 '13 at 20:11
  • @Anuar Then how does conservation of energy work? If two objects apply an equal but opposite force on the other, then kinematics and experiment tells us, the objects will not move since there will be no net force. – ThisIsNotAnId May 04 '13 at 20:14
  • Conservation of energy? Well, I don't get the point. But when you say "net force" you're missing the object. I mean, you should say "the net force upon the object 1". In other words, if you want to know if an object is moving, you should analyze the forces that are exerted upon it (i.e. free body diagram). – Ana S. H. May 04 '13 at 20:18

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Newton's third law doesn't imply that things can't move but it does imply conservation of momentum and energy.

Imagine a scenario where an astronaut is in orbit so they don't feel the affects of gravity. If there is an object floating and they push on it (apply a force to it) we know intuitively that the object will start to accelerate in the direction of the force.

Newton's third law tells us that the astronaut will also start moving in the opposite direction of the force they are applying.

In turn, as the object and astronaut move they push air molecules which push more and so on. If you follow this reasoning through you'll see that ultimately applying a force conserves the total momentum and energy of a system but does not prevent movement of objects within the system.

Brandon Enright
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  • Excuse me, but conservation of energy is fulfilled if and only if the net external force is zero. – Ana S. H. May 04 '13 at 20:22
  • @Anuar exactly. Energy is only conserved for the entire enclosed system which I tried to make somewhat clear with my 4th paragraph. – Brandon Enright May 04 '13 at 20:24
  • Yes, I got it. My question is, how do you obtain conservation of energy and conservation of momentum from the 3rd law? I've seen the derivation of those conservation laws using 2nd, but never only 3rd law. – Ana S. H. May 04 '13 at 20:29
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    Gotcha. Yes, all I do is wave my hands in my answer. Wikipedia says "Newton used the third law to derive the law of conservation of momentum" and I don't think Newton addressed energy. I would like to see a detailed treatment of this too. The second law tells you how momentum changes with force. The third law tells you that the momentum change of one body is perfectly offset by the momentum change in the second body. My reasoning is probably naively similar to the one Newton used. – Brandon Enright May 04 '13 at 20:40
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You must have to think about the mass of the object. Same force will be applied on two different objects of different mass. Velocity (movement) will depend on the mass. Whose mass is more it will be at greater velocity than other higher mass object.

Let's say, i jump on the earth. Same Force is applied both on earth and me. Since my mass is very less compared to the earth, i will move (got velocity). And the same force is two small for the earth two move.

Another example a train stroked a flee. A trained applied a very little force to the flee since the mass of the flee is so small.

sayem siam
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    True, but you don't get the point: even two objects of the same mass can move and interact. – Martino May 04 '13 at 20:53
  • So, you are saying that when a ping pong ball hits the ground it should not bounce because the the force applied to the ground is equal to the same repulsive force by the ground. But the bouncing depends on the type of material the ball is made. Say the rubber ball bounces more than the cricket ball because the rubber ball lower part deforms when the ball hits the ground and another force is applied when ball gets back to its original form and particularly that (another) force is the reason to get bounce. And just same reason why the two objects of the same mass can move and interact – sayem siam May 10 '13 at 16:14