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B Please enlighten me on newton's 3rd law

  1. Mar 3, 2016 #1
    So here it goes. If I am pushing an object, say is shelf. I apply a force of 10N on it, it accelerates, and it exerts 10N back on me. I understand this, but correct me if i am wrong.
    Now, the shelf also exerts a force of 10N on the air particles while it is moving. Shouldn't the air particles also exert 10N back on the shelf, thereby stopping it?
    Please kindly explain in simple terms... Thank you!
     
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  3. Mar 3, 2016 #2

    A.T.

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    What does the 2nd Law say about it?
     
  4. Mar 3, 2016 #3
    In the beginning, the shelf will indeed accelerate, but as the velocity increases, the resistance from the air particles will also increase. At some point, the air particles will indeed push the object back with a force of 10 N, but since you are also pushing the object with a force of 10 N, the object will have an acceleration of zero. That doesn't mean the object stops though! It simply means the speed will stay constant from that moment onward!
     
  5. Mar 3, 2016 #4

    Simon Bridge

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    If the shelf applies a force of 10N to the air in front of it, then the air does indeed return 10N to the shelf ... applying the 2nd law, the forces on the shelf are you, pushing at 10N and the air pushing the other way -10N ... so the net force is zero and there is no acceleration. But that is only if...

    Since you can push a shelf through the air, it follows that the shelf does not push on the air with the same force as you are pushing on the shelf.
    That is just not how air resistance works.

    OTOH: if the shelf were against a wall, then the shelf will push against the wall with the same force as you push on the shelf.
    The main difference here is that the wall is solid, while the air is fluid: the air can get pushed out of the way with little effort.

    Also see post #3 ... at sufficient speed the air resistance may get to be as big as the applied force.
    In order to bring the shelf to rest, though, the air resistance has to be bigger than the applied force.
     
  6. Mar 3, 2016 #5
    i dont get what you mean if i can push a shelf through air, force of shelf on air is actually not 10N?
     
  7. Mar 3, 2016 #6

    jbriggs444

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    Right. The force of the shelf on air is not 10N.
     
  8. Mar 3, 2016 #7
    why?
     
  9. Mar 3, 2016 #8

    Simon Bridge

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    Why would you expect it to be?
     
  10. Mar 3, 2016 #9
    If you can accelerate the shelf through air, that means that the force of the shelf on the air is not equal to 10 N. Why? Third law! (Force of the air on shelf would also be 10 N) If the force of the air on the shelf is 10 N and the force of you on the shelf is also 10 N, the net force on the shelf would be 10 N - 10N = 0 N. The second law then states there is no acceleration.
     
  11. Mar 3, 2016 #10

    Dale

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  12. Mar 3, 2016 #11
    but what about the normal force, which is the reaction force of air on shelf, after the shelf acts on the air? Or is it if i exert 10N on the shelf, the shelf absorbs some and only exerts part of 10N on the air?
     
  13. Mar 3, 2016 #12

    Dale

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    The normal force is between your hand and the shelf and has nothing to do with the air. The force between the hand and the shelf depends on how hard you are pushing on it. The force between the air and the shelf depends on how fast the shelf is moving through the air. Those forces have nothing to do with each other directly.
     
  14. Mar 3, 2016 #13
    You exert a force on the shelf of mass" M". It accelerates to a value "a". Suppose the shelf strike another object of mass " m" where m<<M. What is the force exerted by the shelf on the object? Since the object has the same acceleration as the shelf and since its mass is much smaller than the shelf the force of the shelf on the object is much smaller.
     
  15. Mar 3, 2016 #14
    The act of moving a shelf is kind of unusual to me, so I'm going to substitute a fan in the example. If you swing a fan with your hand, you exert some force on the fan, and the fan exerts some force on the air. The air exerts force back on the fan. You feel this force as an added resistance when you move the fan. But you exert more force on the fan than the air does, so the fan moves.

    If you tried to swing the fan, but the fan was frozen into a huge block of ice with just the handle sticking out, assuming you weren't strong enough to move the block of ice, then you would be exerting force on the fan, the fan would exert force on the ice, and the ice would exert equal force on the fan, and the fan would exert equal force on your hand, so nothing moves.
     
  16. Mar 4, 2016 #15

    russ_watters

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    I do like the word "absorbs" here -- it is why you have unbalanced forces. But it has to go somewhere, so what happens to that force? What does a net force do?
     
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