1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newton's 1st law doubt

  1. Aug 20, 2010 #1
    Hello friends! I'm new to this site and can see its good.
    Well I have a doubt which I would like to get clarified.

    According to newton's 1st law ,
    Any body continues to be in its state of rest unless and until an external force acts on it.

    Imagine a box kept on the floor.
    There are 2 forces acting on it.
    1)gravitational force ie downwards.
    2) upward thrust by the floor.

    So according to newton's 3rd law,
    For every action there is an equal and opposite reaction.

    How do we know it is equal?What is the proof?

    Assume that the magnitude of gravitational force is greater,then the box should start moving in the direction of the gravitational force.ie downwards.

    Am I right?
    Please help.
  2. jcsd
  3. Aug 20, 2010 #2
    Re: doubt

    Yes. Like some object in water, denser than the water. But there is drag also so the object will have some terminal velocity.
  4. Aug 20, 2010 #3
    Re: doubt

    Well then how do you prove newton's 3rd law?
    Assuming the gravitational force is greater than the upward thrust so the box starts moving downwards.

    Like can the magnitude of gravitational force be higher than upward thrust?
    If no why?
    If yes why?

    Please help.
  5. Aug 20, 2010 #4
    Re: doubt

    I dont think I know about any proof about newtons laws(but they are accepted by expts) but I would like to say something..

    The upward reaction IS NOT the reaction to the gravitational force .. hence it may or may not be equal! The actual reaction is the gravitational force that the box exerts on the earth.

    Hence magnitude of gravtational force may be greater the upward reaction or maybe not. (eg just like vlado said)
  6. Aug 20, 2010 #5
    Re: doubt

    The third Newton law states that when two objects interact they exert equal and opposite forces on one another. This means that whenever a first object exerts a force F on a second object, the second object exerts a force −F on the first object.
    The water can oppose only with some maximal force on the object (that depends on the its density ) lets say 100 N. So the object can only apply 100 N on the water. If the weight of the object is lets say is 150 N, it will have extra 50 N to accelerate with, to its terminal velocity.
    But the action-reaction forces between the object and the water will be the equal, and the object will accelerate downwards.
  7. Aug 20, 2010 #6
    Re: doubt

    Because a bizillion perpetual motion machines have failed to work.
  8. Aug 20, 2010 #7


    User Avatar

    Staff: Mentor

    Re: doubt

    Get a room with a glass floor. Place the box on a scale and read what it says. Then flip the scale upside down and read what it says again. If the readings are equal, the forces are equal. :biggrin:
  9. Aug 21, 2010 #8
    Re: doubt

    That's a very flip-ant answer! :redface:

    To the OP - Newton's 'laws' are postulates. That is, things taken to be obviously true and therefore requiring no proof. They are just mathematical re-statements of the conservation of energy and momentum.

    In fact, it's mathematically possible to create an entirely new set of 'laws' based on a new definition of force that says 'the action is equal to half of the reaction' or it's square or anything you like.
    But if you do that, the price you pay is that all the other laws become extremely complicated.

    It's much the same situation as with the old view of the earth as the centre of the universe. All the astronomical 'laws' became so complicated that it was almost impossible to calculate simple things like the orbit of a planet. When we moved to a sun-centred view, things instantly became easier.

    So, the answer is - because it's easier that way.
  10. Aug 21, 2010 #9


    Staff: Mentor

    Re: doubt

    In mechanics Newton's third law embodies the conservation of momentum. So any experiment which violates the conservation of mechanical momentum will falsify Newton's third law.

    It is not just accepted for convenience, there are centuries worth of experimental data including freshman lab experiments.
  11. Aug 21, 2010 #10
    Re: doubt

    If you have an object, sitting on the floor, it doesn't move up or down because although there is a downward force acting on it (gravity), there is also a presumed force acting upward.
    We are forced to presume the upward force because without it the object would be required to move (by the first and second laws).We are then safe to say the two forces are equal and opposite and that's why it doesn't move.

    However, you could do something else - for example, you could invent a new law that 'forces acting on a body in contact with another pass through that body without effect and act on the other side'. That works too.

    Newton's third law is not experimentally verifiable because it is simply a convenient postulate.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook