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Newtons third law

  1. Jun 29, 2012 #1
    Please anybody explain me ... if i punch a soft cushion hardly say "F" force will thw cusion hurt me with the same ... every body says "NO" ok all right everybody says "you, in this case have not applied the "F" force because most part of it is wasted in changing the shape of cusion " so here comes the question what happens with the newtons third law???
    is it not meant for the things like cushion??
    please explain ??
  2. jcsd
  3. Jun 29, 2012 #2

    Doc Al

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    Staff: Mentor

    Newton's 3rd law applies just fine. Since the pillow is light and soft, you simply cannot punch it hard. But whatever force you do manage to exert on it, the pillow will exert an equal and opposite force on you.
  4. Jun 30, 2012 #3


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    Science Advisor
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    When you punch something you initially accelerate your hand and arm to a velocity. Lets call that Vpunch. When it hits the object your hand and arm have to decelerate from Vpunch to zero again.

    If you know the stopping distance "s" you can can apply the equation of motion

    V^2=U^2 + 2as

    to work out the deceleration "a".

    Then if you know the mass of your hand and arm "m" you can work out the force the object needs to apply to your hand and arm to slow it down using


    The stopping distance "s" for a cushion is much longer than that of a wall. Than means "a" and "F" is lower for a cushion than a wall.

    Doc Al is also correct. Newtons law means that at all times the force your hand applies to the object is the same as that which the object applies to your hand.
    Last edited: Jun 30, 2012
  5. Jul 3, 2012 #4
    thank i got it. "Cwattera and Doc al"
  6. Jul 3, 2012 #5
    please tell me how can i post a new threat ....i am new for this site please help...
  7. Jul 3, 2012 #6

    Doc Al

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    Staff: Mentor

    Check out our Physicsforums FAQ.
  8. Jul 3, 2012 #7
    ok now i got it thanks
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