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

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Discussion Overview

The discussion revolves around the mechanics of ball collisions, specifically why a ball moves at an angle after colliding with another ball, such as in billiards. Participants explore the forces at play during the collision and the resulting motion of the balls, addressing both theoretical and conceptual aspects of the interaction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the second ball moves perpendicularly to the point of contact due to the nature of the contact forces acting on the balls.
  • Others argue that the force applied during the collision is perpendicular to the surfaces of the balls at the point of contact, leading to the observed angle of motion.
  • A participant questions why the force should not act in the direction of the first ball's motion, suggesting that the movement should align with the applied force.
  • Some participants clarify that the normal force, which is perpendicular to the contact surface, is responsible for the direction of the second ball's movement.
  • There is a discussion about the intuitive understanding of forces and how they apply during collisions, with some participants expressing confusion about the relationship between the direction of force and the resulting motion.
  • A participant proposes that electronic repulsions at the point of contact could explain why the red ball moves perpendicularly to the contact surface.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the intuitive understanding of the forces involved in the collision. While some agree on the role of the normal force, others express confusion about why the force does not act in the direction of the first ball's motion.

Contextual Notes

Some participants reference figures and diagrams to illustrate their points, indicating that visual aids may be necessary for clearer communication of the concepts discussed. There is also mention of assumptions regarding the smoothness of the balls affecting the nature of the forces during the collision.

  • #31
Doc Al said:
And the answer to that, which I thought I gave a while back, is that we are assuming that the balls are smooth--no friction. Thus any force they exert can only be perpendicular to their surface.

Yep. I think that's the crux. But I'm not sure it's entirely obvious why that's so.

I've been trying to come up with a macro-scale example of poking an object such that you can't impart any directional momentum on it. Like pushing a floating boat with your finger, or tapping a balloon.
 
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  • #32
DaveC426913 said:
Yep. I think that's the crux. But I'm not sure it's entirely obvious why that's so.

I've been trying to come up with a macro-scale example of poking an object such that you can't impart any directional momentum on it. Like pushing a floating boat with your finger, or tapping a balloon.

Yea I think that's where the confusion is coming from. Right now it's more like we are told this is true without explanation.
 
  • #33
Red_CCF said:
Yea I think that's where the confusion is coming from. Right now it's more like we are told this is true without explanation.

Ok let me take a stab at it.

The cause of the normal force is electromagnetic repulsion between atoms on the two bodies, yes? Since there is no friction, then we are assuming that the surfaces are smooth at an atomic scale. Hence, if we zoom into the area of contact between the two bodies, it would look like perfectly flat parallel surfaces. So there can only be a force perpendicular to the two surfaces.

Does it make any sense?
 
  • #34
DaveC426913 said:
Yep. I think that's the crux. But I'm not sure it's entirely obvious why that's so.

I've been trying to come up with a macro-scale example of poking an object such that you can't impart any directional momentum on it. Like pushing a floating boat with your finger, or tapping a balloon.
I think this is just way too obvious to need a "why". Why does a car on ice spin its wheels instead of moving forward? Because there is no friction between the wheels and the ground. Is this not obvious to you?
 
  • #35
The cause of the normal force is electromagnetic repulsion between atoms on the two bodies, yes? Since there is no friction, then we are assuming that the surfaces are smooth at an atomic scale. Hence, if we zoom into the area of contact between the two bodies, it would look like perfectly flat parallel surfaces. So there can only be a force perpendicular to the two surfaces.

Does it make any sense?
Yeah, that makes sense I think. I also thought that!
 

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