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When regarding Newton's 3rd law, I have a question. It seems like while I study, there is no reaction, the actions act at the same time. Is this correct? And also, I really don't understand why something would "push" back at you. I understand that it happens because of real life examples, I just need clarification on how something can push back. I know this is really basic, I would just like some help. Thanks!

Lightbulb
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  • A technically incorrect, but physically close and understandable to layman explanation, would be to think of magnets. When you push on a desk, imagine at the microscopic level your hand is a bunch of tiny particles that act like magnets. The desk is comprised of the same tiny magnets. When they come in close contact they repel each other. A lot of interactions in our life are these electromagnetic forces, pushing on each other. Why they are equal and opposite? One way to think about it is an isolated system cannot change it's center of mass coordinate, i.e., conserves momentum. – Novice C Feb 26 '16 at 01:17
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    Thinking about reaction forces can be confusing. Maybe think about it this way: a force is an exchange of momentum. When you push on something, it gains momentum, and at the same time you lose it. So forces must come in pairs. – knzhou Feb 26 '16 at 01:19
  • Most likely you will learn about conservation of momentum as a result of newton's third law, and what I said is circular. In reality conservation laws have a much simpler and fundamental reason, i.e., symmetries, which in turn gives us Newton's third. So when learning these laws it is easier to accept them as postulates, and therefore are not to be questioned. However, it is done so at the elementary level for good reason. – Novice C Feb 26 '16 at 01:22
  • The action-reaction forces act on different bodies; the force from A acts on B; the force from B acts on A. Keeping this straight avoids most problems. – Peter Diehr Feb 26 '16 at 02:50

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I think you are just wondering how any body which is inanimate, i.e. material objects can provide reaction force opposed to the action by us humans.

These forces of action and reaction are generated in the material also and is visible when bodies are in equilibrium with each other i. e. no relative motion is being observed.

The reaction forces are always normal to the plane of contact in case of action in contact.

A body which is pulling another one ;say a box on earth -being pulled by gravitational force towards its center is pushed by the earths surface opposite to the action and equal in magnitude otherwise the box can not be at rest- at rest means net force acting on it is zero.

However if the earth's surface fails to provide equal reaction(in case of marshy land) then the body will move by the difference of these two forces and it will go down in the surface till the hard surface(equal reaction to the weight of the body) is reached. thats why you go down on a sandy surface- beach surface- as the particles of sand are not tied as atoms to each other and they slip.

The mechanism of force of reaction is that when you apply a force say on a block of wood, you are trying to deform the arrangement of its atoms which are in an equilibrium under inter atomic forces - if you disturb their arrangement their inter atomic interaction will push you - they do not like being disturbed- you are actually pushing the atoms closer then the inter atomic forces will push you back- why they do that ; imagine the atoms to be at d distance in equilibrium -if you push to reduce d the net force will push you back to go to equilibrium ; if you try to pull them apart they will again pull you to get to their minimum energy state i.e. equilibrium arrangement . every natural system wants to stay in minimum energy state called ground state.

You have a steel wire hung from a rigid support- you hang a weight W . now this weight w will be trying to change the atomic arrangement of the wire , which were in equilibrium- the tendency of the arrangement to come back to ground state gives a pull on the w -this is reaction- now you increase the weight W+ dw - perhaps you have pulled the atoms apart as when you measure the length -you get an increase in length -now you remove the weight dw - you will find the change in length goes to zero - i.e. you are in elastic limit of the wire . so things like to go to their minimum energy configuration. But if you cross the elastic limit to get an equal and opposite reaction the steel becomes plastic-i.e. its change in length does not return to original length and ultimately the wire breaks at certain Weight W(1) -that is breaking weight.

drvrm
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  • So action-reaction happens because of the interaction between atoms of both objects. The system wants to stay at the lowest energy state, so the other atoms either push/pull back by atomic forces to stay in the low energy state. Do I have this right? – Lightbulb Feb 26 '16 at 13:21
  • Not necessarily there are various forces giving rise to a action reaction pairs- for example gravitational forces,electromagnetic force body ....If a body A is pulling B say by gravitational force the body B also pulls A by an equal amount. similarly between magnetic bodies...there can be several examples. – drvrm Feb 26 '16 at 14:29