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B Motion on a frictionless Surface

  1. May 25, 2017 #1
    "Suppose a body is kept on frictionless surface (considering gravity is acting on a body). If we give a little push to the

    body, will it conitnue to be in state of motion with constant velocity or not?...If yes then why?? (Considering all

    resistances to be zero ..)
     
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  3. May 25, 2017 #2

    hilbert2

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    If the surface is perpendicular to the direction of gravity then yes, because of Newton's 1st law.
     
  4. May 25, 2017 #3
    Yes, it will do so.
    Newton's first law says that a body in motion WILL remain in motion forever unless you give it a small amount of force.
    Now normally friction opposes motion and so bodies do NOT remain in motion, forever.On a friction less surface, however, there is nothing to stop or oppose the motion of the body.So, the body will go on and on and on.
     
  5. May 25, 2017 #4

    sophiecentaur

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    For direction or speed (=velocity) to change, there has to be an external applied force. In your scenario, there is no applied force.
    An historical note: People 'before Newton' observed that everything slows down and that was a pretty accurate observation for everything they saw on Earth. That was because of friction, of course. They looked up into the sky and saw the stars etc. moved around without slowing down so they assumed that there had to be an engine making it happen. Wrong. It's just that the friction is so low up there. Newton neatly managed to tie together the heavenly and earthly behaviour with his laws.
     
  6. May 25, 2017 #5

    Nidum

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    So what happens if the surface is frictionless and nominally level but imperfect - ie with lots of small humps and hollows ?
     
  7. May 25, 2017 #6

    sophiecentaur

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    If it comes to a hill, it will slow down (losing kinetic and gaining potential energy) and, on the way down the other side it will speed up, (losing the PE and gaining the KE it lost) The effect of gravity takes no net energy from the system if it regains the original level. If the track went right round the world, the same thing would apply.
     
  8. May 25, 2017 #7

    russ_watters

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    I think you just said frictionless and level but not frictionless or level.
     
  9. May 25, 2017 #8
    When it encounters a small bump, there would be a component of force opposite to the direction of motion of the object, tending to slow it down a little. It will eventually come to a halt if there are enough imperfections on the surface.
     
  10. May 25, 2017 #9

    sophiecentaur

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    'Fraid not. In the absence of friction, there will be no energy lost. Merely deflecting the body will not take energy from the system. Read what I wrote above. We are discussing a non-intuitive ideal situation.
     
  11. May 25, 2017 #10
    We usually distinguish between friction (of objects in contact) and elasticity of collisions. If the collisions are perfectly elastic, then as you go over some bumpy terrain, the object will possibly start bouncing around, since there's no dissipation to keep it stuck to the ground. If the collisions aren't elastic, then you can lose some energy going over rough terrain even if the contact friction is negligible.
     
  12. May 25, 2017 #11

    sophiecentaur

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    Yes. It all depends on how real you want to get.
     
  13. May 25, 2017 #12
    I was thinking of a small bump with ~ vertical sides. But by definition if there is no friction then there are no such bumps.
     
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