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I How does anything slow down if kinetic friction is low?

  1. May 27, 2016 #1
    Lets say that there is a rectangular box on a horizontal surface, with no air resistance on it. If the applied force required to get the object moving is greater than the static friction, Fa>Us* Fn, then the object starts moving. However, the Static Friction is almost always greater than the Kinetic Friction, Us> Uk. That means that the applied force is more than the resulting kinetic friction from sliding: Fa > Uk. If so, then how does an object decelerate if the force is positive (in the direction of movement)?

    In summary, how does an object stop moving if the sliding friction is lower than the applied force (such as a book sliding down a table)? A force only changes the acceleration. In order to decelerate, the kinetic friction must exceed the applied force and therefore, static friction as well.
  2. jcsd
  3. May 27, 2016 #2


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    If you continue to apply the accelerating force, the object will accelerate, not slow down.
  4. May 27, 2016 #3
    It won't slow down if the applied force is applied continuously. Neither will it stop. Why do you think it will stop?

    In the situation you mentioned (book sliding down on a table) you give it a force [itex] F > \mu_s n [/itex] it sets the book moving /sliding down the table. However once it starts moving , it loses contact with you and the force stops acting on it. While it is moving the only force acting on it is kinetic friction / sliding friction (of course, gravitational force and normal force are acting on it!) in the direction opposite to it's motion which causes the book to stop.

    In summary if a force is applied continuously on a body in the direction of motion [itex] >= [/itex] the friction it won't stop moving.
  5. May 27, 2016 #4
    Then why do things slow down, even though the applied force appears to be lower than the kinetic friction? Is it because of drag?
  6. May 27, 2016 #5


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    Because you stop applying the accelerating force. If you don't the object will not stop.
  7. May 27, 2016 #6
    But you only need to apply the force at first to accelerate it, after which it should continue to accelerate unless acted on by an external force (the kinetic friction is lower than the force so it should only decrease the acceleration, not cause the object to decelerate).
  8. May 27, 2016 #7
    Sorry, previously when I said:
    I meant that the kinetic friction appears lower than the applied force.
  9. May 27, 2016 #8
    That is a fundamental mistake. If a force stops acting on it the body will continue to move with the velocity gained not accelerate. Acceleration is the result of a force. If a force stops acting on it , the body stops accelerating unless another force acts on it.
  10. May 27, 2016 #9
    Thank you for explaining that, but could you please give me a citation for it?
  11. May 27, 2016 #10
    That is Newton's first law:

    An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.


    The first law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant.
  12. May 27, 2016 #11
    I believe you distorted Isaac Newton's actual words quite a bit. If your version were true, if you threw a ball, it would barely accelerate due to the short amount of time your hand was in contact with it (the short amount of time you applied the force). However, if what I said earlier were true, then the high amount of air resistance along with the low inertia of the ball would eventually make the ball slow down, although the ball was still 'trying' to accelerate. I think he said this: "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it."
  13. May 27, 2016 #12
    I don't see how my statement is different from yours. Yes, the ball in my hand only accelerates in the amount of time it is in my hand but the acceleration causes the ball to gain a velocity which carries the ball upwards/forward till gravity and air resistance slows it down and eventually stops it.
  14. May 27, 2016 #13
    Actually he wrote neither. In his book it was like this:
    Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

    Not that is relevant to your confusion but is good to remember that some things are translations and not the actual words of the original author, especially when you ready to argue about who distorted "his actual words".
  15. May 27, 2016 #14
    Thanks, I was looking for the actual statement.
  16. May 27, 2016 #15


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    No. After you stop applying it the force is zero. So the only force acting on it is the kinetic friction, which acts in the direction opposite the motion causing the object to decelerate.
  17. May 27, 2016 #16


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  18. May 27, 2016 #17


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    Just to clarify, did you mean a book sliding across a horizontal table, or a book sliding down an inclined table (down a slope)?
  19. May 28, 2016 #18
    this will happen in either of 2 ways
    1) set object in motion and stop applying force . object retards
    2) set object in motion(greater force than static friction) and now apply force smaller than kinetic friction in dir of motion of object, object retards
  20. May 28, 2016 #19


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    It will not decelerate/slow down unless something changes. As long as the net force is positive (in the direction of movement) it will accelerate.
    You said "down" a table so I assume this is a sloping table...

    If you put a book on a horizontal table and then gradually change the angle there will come a point where the book starts to slide. If kinetic friction is less than static friction the book will accelerate until it falls off the table.

    If that does NOT happen, say if the book slides for a short distance and stops then the only way that can happen is if the coefficient of kinetic friction is different at different places on the table (eg it hit a sticky spot) or the table isn't dead flat (so the angle changes).
  21. May 28, 2016 #20
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