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

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SUMMARY

The discussion centers on the mechanics of billiard ball collisions, specifically why a ball moves at an angle after colliding with another ball. Participants explain that the force exerted during the collision is the normal force, which acts perpendicular to the surfaces at the point of contact. This normal force is responsible for the angle of movement, as it does not align with the direction of the incoming ball's motion. The conversation highlights the importance of understanding normal forces in elastic collisions and how they dictate the resultant trajectories of colliding objects.

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  • Understanding of basic physics concepts such as force and motion.
  • Familiarity with normal force and its role in collisions.
  • Knowledge of elastic collisions and momentum transfer.
  • Ability to visualize and analyze vector components in physics.
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  • Study the principles of normal force in physics, particularly in collision scenarios.
  • Learn about vector decomposition and how forces act at angles during collisions.
  • Explore elastic collision equations and their applications in real-world scenarios.
  • Investigate the effects of friction on collision outcomes and how it alters trajectories.
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Physics students, educators, and anyone interested in understanding the dynamics of collisions in mechanics, particularly in sports like billiards and snooker.

  • #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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