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Vector force?

  1. Apr 1, 2003 #1
    vector force???

    Hi all. I'm in search of a simple answer to a simple question that has become seemingly impossible for me to find. I'm not schooled in Physics. I can cypher fairly well, but this doesn't really(I think)require cyphering.

    It's a question of golf club golf ball collision. Elastic. All I have found on elastic collision is in-line collision.

    If a club, say with about 10° of loft hits a golf ball at ~100 mph, would the angle of the clubface dictate direction of travel or would the swingpath. I'm not asking about the loft. I'm asking about an angular impact; the clubface going straight at the ball with 5°, 10°, 15° of angle to impact direction.

    The reason I ask is there are two schools(ha)of thought on this. One says the ball will travel in the direction of swingpath and spin(and therefore curve) because of clubface angle.

    The other says the ball will travel in direction of clubface angle(mainly-I realize there is probably an element of path involved)and spin(curve) because of angular impact.

    Anybody know for sure?

    Last edited: Apr 1, 2003
  2. jcsd
  3. Apr 1, 2003 #2


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    Staff: Mentor

    Both are right - the initial direction is determined by where the ball is struck and the angle of incidence of the club. Irregularities in the swing (if it isn't lined up with the ball's initial direction) cause it to spin.

    Think about billiards balls for the direction question. When you play pool, where do you want the balls to make contact? You want them to mke contact on the exact opposite side of the direction of intended motion.
  4. Apr 1, 2003 #3
    Thanks Russ for your reply. I like the billiard ball analogy. It made me think, then things got fuzzy.

    Let me ask a simple question.

    If a block of steel cut to a 20° angle on the leading edge was propelled into a ball along a straight line, what angle would the ball take off in?

    Would the 20° component be more influential than the straight impact direction.

    I would think it would be closer to 20° than straight ahead, maybe 17-18°.
  5. Apr 1, 2003 #4


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    Staff Emeritus
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    Gold Member

    Think relativity! (the gallilean kind, not special relativity).

    Ignore gravity for the moment.

    In one reference frame you have the inclined plane sliding towards the golf ball...

    But if you choose another reference frame, you have a stationary inclined plane with the golf ball hurtling towards it.

    From there you can apply the law of reflection, that the angle of incidence is equal to the angle of reflection... so the ball's new path is reflected across the normal line. Since the ball is hurtling towards our flat plane at 20 degrees to the normal, the ball is reflected to 20 degrees on the other side... the net reflection is 40 degrees!

    Then we shift back to the original reference frame with the stationary golf ball, and the conclusion carries over, if the block has a leading edge 20 degrees off of normal, hitting the golf ball will cause it to take a trajectory with an initial 40 degree angle.

    Putting gravity back into the picture, I'm not entirely sure as to what the net effect would be. I imagine it would cause the initial angle to be somewhat less than the no gravity case (because some of the impulse of collision goes towards replacing the normal force), though the higher the velocity of impact the less effect gravity has, so a high speed impact would cause the golf ball to jump to a near 40 degree trajectory, but a low speed impact wouldn't get it off of the ground.

  6. Apr 1, 2003 #5
    Thanks Hurkyl. Reflection. Why didn't I think of that? Please don't answer that.
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