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Hi I have a quick question. I know this may be a common question. If N3L states that there's an equal and opposite force, why do things still move?

I have read up a lot and this is my understanding. If a finger exerts a force on a box, then box will move/accelerate. At the same time, box exerts an equal force on the finger. The motion of box(assuming no friction) is affected only by force from the finger. (I think the term is System = box). Thus, the reaction force from box only affects the motion of finger and has no relevance to the motion of the box.

However, I came to this question. If the reaction force from the box on the finger is equal to the force from the finger to the box, then there must be some other kind of force on the finger that is opposite to the reaction force and greater, since the finger is accelerating/moving. What is that force?

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marked as duplicate by Kyle Kanos, Martin, Rob Jeffries, BMS, John Rennie Jan 20 '15 at 6:57

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Correct.

You need to make a free body diagram of your sliced off fingertip to realize that there is an internal force within your finger that is being applied to your disembodied finger tip. That is what is causing your fingertip to accelerate with the box.

The whole crazy scenario must come to an end when you can't run fast enough to maintain the acceleration of the box and your fingertip.

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  • $\begingroup$ So that means the internal force is not equal to the force transmitted from the finger to the box? I always had an understanding that all the force from the hand is transmitted fully to the object that it is pushing. $\endgroup$ – Zhi J Teoh Jan 19 '15 at 22:25
  • $\begingroup$ Correct. The new free body system would be the box PLUS your fingertip. That is a greater mass than just the box alone that is accelerating at the same rate as the box alone. Follow the chain to the floor. The force acting on your feet as you run to maintain the acceleration of the whole system is now much greater than your resting body weight. $\endgroup$ – Inquisitive Jan 19 '15 at 22:28

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