# Need help understanding Newton's third Law

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#### September24

##### Full Member
10+ Year Member
I know that newton's third law talks about "equal and opposite" forces, but what does it mean that:
"action-reaction forces act on different bodies"

What limitations does this fact provide? I'm having trouble understanding it aside from that an action force will act on an object and the object will provide a force on something other than itself.

Example problem:

"Newton's Third Law states that 'to every action, there is an equal and opposite reaction.' This means that a rope will pull on a mass with same magnitude of force that the mass pulls back on rope. Why is the mass able to move from rest when a tension is applied to an attached rope?"

A. Force exerted on the block is actually larger than the force on rope.
B. It has nothing to do with rope and everything to do with a friction force.
C. The person pulling exerts a larger force on the rope.
D. The action-reaction forces act on different bodies.

Answer is D. How does it apply to the problem.

Ok, so let us say you are standing on ground. There some force that you are exerting on the ground due to gravity, correct. So where is that force acting. We will say that force is acting on the ground, not on you. Now Newton said in response to the force you exert on the ground the ground exerts and equal amount of force on you, the person. That force is acting on you not the ground. So if that be the case well then what is weight? Is it the force you exert or is it the force earth exerts on you? Weight is defined as the force exerted by you on earth, due to gravity. Well then what about the force earth exerted on you? That is called Normal force and in this case, normal force is same as weight. At least that is my understanding.....so that is why we say, action and reaction pairs are equal and opposite and act on different bodies.......sorry.....totally forgot about your question. So with this understanding in regards to your question, we can say the mass exerts a tension T on the string and the string exerts a similar tension T'''' on the mass. The reason mass moves is because it only experiences the tension T from the rope. The force that the mass exerts is felt by the rope not the mass, hence the mass moves.....

Ok, so let us say you are standing on ground. There some force that you are exerting on the ground due to gravity, correct. So where is that force acting. We will say that force is acting on the ground, not on you. Now Newton said in response to the force you exert on the ground the ground exerts and equal amount of force on you, the person. That force is acting on you not the ground. So if that be the case well then what is weight? Is it the force you exert or is it the force earth exerts on you? Weight is defined as the force exerted by you on earth, due to gravity. Well then what about the force earth exerted on you? That is called Normal force and in this case, normal force is same as weight. At least that is my understanding.....so that is why we say, action and reaction pairs are equal and opposite and act on different bodies.......sorry.....totally forgot about your question. So with this understanding in regards to your question, we can say the mass exerts a tension T on the string and the string exerts a similar tension T'''' on the mass. The reason mass moves is because it only experiences the tension T from the rope. The force that the mass exerts is felt by the rope not the mass, hence the mass moves.....
Ignore my earlier blog. I was wrong in my understanding. That is not a correct explanation of action reaction pairs. However I did find this example which is a good explanation of how action reaction pairs happen.

http://sun4.vaniercollege.qc.ca/VirtualMentor/physics/action_reaction.html