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Why does a ball moves at an angle when it collides with another ball?

  1. Jun 1, 2010 #1
    Hi friends
    Consider two balls such as billiards balls . When one ball collides with another ball at a point which is some distance away from centerline of two ball system, both balls move at some angle relative to each other depending upon the point of contact during collision.
    Why does this happen?
    Why don't the second ball moves straight after getting hit by the first ball?
  2. jcsd
  3. Jun 1, 2010 #2
    The second ball is just moving perpendicularly to the point of contact an any case.
  4. Jun 1, 2010 #3
    The contact forces between the billiard balls act perpendicular to the balls' surfaces at the point of contact. Sketch the collision and draw the forces, and you'll see why they move as they do.
  5. Jun 1, 2010 #4
    the ball moves prependicular to the tangent at the point of contact. Why?
    It should move in the direction of force applied i.e purely vertical

    Attached Files:

  6. Jun 1, 2010 #5
    The force applied is perpendicular to the contact point, not in the same direction as the ball is travelling. This is why it's hard to cut a snooker ball when it is close to 90 degrees.
  7. Jun 1, 2010 #6
    Maybe if you stated why you think there would be a force component parallell to the contact point, it would be easier to clear any misconceptions.
  8. Jun 2, 2010 #7
    Why do you think of parallel force component ? Force is applied in vertical direction and so should the red ball move in vertical direction.
  9. Jun 2, 2010 #8
    Why do you think the force would act in the direction of movement?
  10. Jun 2, 2010 #9
    When did i say this? I said that movement should occur in the direction of force applied.
  11. Jun 2, 2010 #10
    The force applied isn't in the direction the ball is/would be moving. It's the normal force that causes movement, and the mornal force is perpendicular to the contact, thats why it moves at an angle.

    You showed a ball moving vertically towards another ball, and asked why the second ball does not move vertically. To me that means you think that the force applied to the second ball is in the direction of the 1st balls motoin.
  12. Jun 2, 2010 #11
    Yes i think same. I know i am wrong because in reality ball moves at angle this means force acts prependicular to the red ball but i don't know why this happens.
    Black ball should apply force in the direction it is moving!
  13. Jun 2, 2010 #12

    Doc Al

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    Staff: Mentor

    But then you say...
    Two very different statements.
  14. Jun 2, 2010 #13
    i accept i said all you wrote above and i know i am wrong just tell me why i am wrong?
    Why don't the black ball aplly force in the direction it is moving??????
  15. Jun 2, 2010 #14
    EDIT: nm you realise that it's the normal force so I'll unsize it.

    This is how all things apply force when in contact with each other. Think of a block on a slope, gravity acts vertically downwards, but the force between the contact is perpendicular to the slope. It's difficult to explain it more simply, as that's just how things contact each other.
    Last edited: Jun 2, 2010
  16. Jun 2, 2010 #15

    Doc Al

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    Staff: Mentor

    Since the balls are smooth, the only force they can exert on each other is normal to their surface. (As xxChrisxx has explained several times.) That force will have a component in the direction of motion of the black ball, but it acts at an angle.
  17. Jun 2, 2010 #16

    Doc Al

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    Staff: Mentor

    Why what?

    Why the force is normal?

    Why the direction of the normal force is not in the direction of motion?

    Why the acceleration of the hit ball must be in the direction of the force?
  18. Jun 2, 2010 #17
    ok i think i got it!
  19. Jun 2, 2010 #18
    that's how bodies interact and forces are applied
  20. Jun 2, 2010 #19


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    Gold Member

    The OP seems to be asking a pretty comprehensible question. I am at a loss at to how it is not being understood.

    Why is the force applied by the black ball acting in a direction normal to the contact surface rather than parallel to the black ball's motion (as might be intuitive to the layperson)?

    I think he gets that the force is applied normal to the contact surface, but why?
  21. Jun 2, 2010 #20
    We all get the question, and we all understand what the op is saying. Care to take a stab at answering that question?

    EDIT: And in the OP it was in no way clear that he understood it to be the normal force acting. It's was only in post 11 after many people had stated it that he agreed it was the normal force.
    Last edited: Jun 2, 2010
  22. Jun 2, 2010 #21
    But i posted a figure in 4th post which i think u didn't see!
    you are wrong!
    If i didn't know that force was "in real" normal then this question would not arise in my mind!
    Secondly, i could not upload the figure i did if i didn't know the force was normal.
    Look at the figure where i draw a common tangent and then i show the actual direction of movement of red ball and the dotted one which i thought should be the direction(i know i was wrong in that).
    I know i was wrong that force should act in the direction of motion of black ball but what i wanted to find out was why?
  23. Jun 2, 2010 #22
    I think you understand me what i wanted to know!
    Now i imagined to find out the answer. Tell me if i am correct:
    A body experiences force and thus acceleration due to electronic repulsions between the body and the body applying force. So when a ball collides with another ball as in my system stated earlier, then at the point of contact electronic repulsions would be prependicular to the point of contact and so the red ball will move in the direction prependicular to the point of contact.
    Correct me if i am wrong!
  24. Jun 2, 2010 #23
    That's fair enough communicating thoughts on a forum is not always clear, i'm rubbish at it.

    But the way you worded the responses was slightly confusing until you explicity stated in post 11 that it's the normal force, even then it was not clear that you understood. Especially when you stated after that that the force should be in the direction of the black balls movement (post 11).

    Then the question became why the normal force? And the only answe I can come up with is "becuase it is". I'm hoping Dave or someone can come up with something more substantial.
  25. Jun 2, 2010 #24
    so what about the answer i think in post 22?
  26. Jun 2, 2010 #25
    Because only in the direction normal to the contact surface force can be "taken" by the second ball.That is the only direction in which the second ball is resisting the motion of the first ball.The coefficient of friction is very very small. If there was bigger coefficient of friction between the balls there would be force in the direction paralel to the contact surface.
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