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Opposing forces on objects in motion

  1. Jul 6, 2015 #1
    1. The problem statement, all variables and given/known data
    A tennis ball is rolled across a carpet. Explain the forces exerted on the ball by the person rolling it and the forces that bring it to rest.

    2. Relevant equations

    3. The attempt at a solution
    The hand exerts an impulse on the tennis ball which causes a change in momentum and then it starts to roll. Air resistance will act on the ball which will slowly bring it to rest. Could someone help me understand if friction applies. I did some reading and apparently you can only ever get friction between the object and the floor if there is some dynamic friction. The reason is that static friction by definition can do no work against the motion of any object so if the object is not moving relative to the surface then friction from the surface can never oppose the motion of an object. Is this right? So if a roller coaster or a car are in motion, so long as the wheels never slip, friction from the surface can never slow the object down? If we assume no dynamic friction on the tennis ball across the carpet then is the only loss of motion through air resistance?
  2. jcsd
  3. Jul 6, 2015 #2
    Well, do you think that air resistance amone would be able to stop a tennis ball ?, (Of course it can, but how long should you hold you hold your breath ?,), Why do you think static friction cannot apply a resistive force again the direction of motion ? What about kinetic friction ?,
  4. Jul 6, 2015 #3
    Static friction cannot slow the ball down. To slow the ball down requires doing work against the ball - Work = Force x Distance. Since this friction is 'static' it cannot oppose the motion of the ball. I don't think there is any kinetic friction because if a ball is rolled across the carpet then there is no relative motion between the carpet and the ball - at least this is what my teacher said (to assume no relative motion, like a car wheel rolling along the road).

    The only other opposing force I can think of is the weight of the tennis ball causing it to slightly squash and stretch as it rolls which would cause heating loss.
  5. Jul 7, 2015 #4
    Static friction does exert force but on an object in rest, because it's "static", once the object starts moving kinetic friction will slow him down, kinetic friction exerts a force opposing the direction of motion while the object is moving, air resistance isn't strong but it can stop a lot of things, you must include kinetic friction !,
  6. Jul 7, 2015 #5


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    What force makes it start to roll?
  7. Jul 9, 2015 #6
    I still have no idea about whether or not friction applies. I was told that a car tyre does not move relative to the road surface so static friction is the only frictional force at work and this provides the driving force (as per Newton's 3rd Law) which propels the object forward. In terms of opposing motion surely work must be done against the motion of the car, i.e. Force x Distance. But static friction cannot do any work against the motion of the car since it is 'static'. Kinetic friction surely cannot apply here at all because this only applies when there is relative motion between the car tyre and the road? This is what I was told anyway and I'm just applying this to the tennis ball situation.
  8. Jul 9, 2015 #7
  9. Jul 9, 2015 #8


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    You may be confusing kinetic friction with rolling resistance. As Jimmy says, there is no kinetic friction here.
    Quite so. This is rolling resistance, and is the main reason the ball slows down.

    You can model rolling resistance by thinking about the compression forces in a partly elastic wheel. At the leading edge of contact, the (vertical) compression force is a little greater than at the corresponding point on the trailing edge. This difference represents the losses in the compression/decompression of the material.
    Draw an FBD of this. What does it tell you about the static friction force?
    Last edited: Jul 9, 2015
  10. Jul 10, 2015 #9


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    It's not clear how much detail they want but I would suggest that the ball cannot start rolling (rotating) instantly as it has moment of inertia. So I'm thinking that kinetic friction with the ground or static friction with the hand starts the ball rolling. Then once it's rolling and not skidding kinetic friction is replaced by rolling and air resistance.
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