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Homework Help: Friction forces on ball hitting wall

  1. Mar 19, 2010 #1
    1. The problem statement, all variables and given/known data

    Suppose I throw a ball at a wall parallel to its x-axis.

    At the moment it hits the wall, what forces are acting upon the ball?

    2. Relevant equations

    None. Relevant assumptions: ignore any spin on ball.

    3. The attempt at a solution

    I think they'll be:
    - gravity acting in the negative y-axis direction
    - friction acting in the positive y-axis direction
    - friction acting in the negative x-axis direction.

    Friction may be kinetic or static depending on whether ball slips while bouncing?

    Have I missed anything? Or included something falsely? I'm pretty sure that there's a friction force in the negative y-axis direction, but not entirely sure, since the ball's path will be arcing downwards with gravity.
  2. jcsd
  3. Mar 19, 2010 #2
    The is no friction in the x direction. That will be your normal force supplied by the wall. Your friction from the wall is in the positive y direction. Other than that I believe that everything is good.
  4. Mar 19, 2010 #3


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    You've forgotten one crucial force, the force that actually makes the ball bounce back.
  5. Mar 19, 2010 #4


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    Why would there be no friction in the x direction? The ball is trying to move in the x direction while the normal force is pressing it against the wall, so there has to be friction.
  6. Mar 19, 2010 #5
    There will be friction because of the normal force but it is in the y direction.
  7. Mar 19, 2010 #6


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    Why not in the x direction too?
  8. Mar 19, 2010 #7
    The wall cant supply friction in the x direction. Its just like when you pull a box on the ground there will be a friction force in the horizontal direction because of the normal force but no component of friction in the verticle direction. I will attach a picture of the FBD in a moment.
  9. Mar 19, 2010 #8
    I'd forgotten about the normal force.

    If the normal force is in the z-axis direction, can the friction force act in any direction on the xy plane?
    Last edited: Mar 19, 2010
  10. Mar 19, 2010 #9
    From the FBD where W=weight N=normal force f=friction one can see that there is no component of friction in the x direction. N and f are impulse forces that will change the balls momentum. If no friction were present the ball would rebound at the same angle it hit at, but with friction it will rebound at a slightly less angle. Also that friction force will apply a torque or moment to the ball in the counter-clockwise direction, but that is not part of the question. The friction at the instant the ball hits is also equal to the shear force experienced by the wall.

    Attached Files:

  11. Mar 19, 2010 #10
    Actaully if it hits parallel to the x axis or with a very small y component of velocity than the friction force will vanish...
  12. Mar 19, 2010 #11
    Something else I forgot to ask:

    Does the ball momentarily stop as it hits the wall? I ask this because I'm wondering if drag acts at all in the instant the ball (which is quite light) hits the wall.
  13. Mar 19, 2010 #12
    Sorry, I'd misstated the original premise.

    The ball is thrown parallel to the ground. It hits the wall while arcing downwards with gravity. It also bounces off the wall at some arbitrary angle to the z-axis.
  14. Mar 19, 2010 #13


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    I can't see that at all. Think about it this way: at the moment the ball hits the wall, some of its molecules lock on to the wall's molecules. As the ball tries to move in the x direction, the intermolecular bonds try to prevent this movement and exert a force in the -x direction.
  15. Mar 19, 2010 #14
    I don't think that molecular forces would come into play for most cases.
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