Why do billiard balls move at angles after an off-center collision?

[DaveC426913]

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.

 

[Red_CCF]

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.

 

[McLaren Rulez]

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?

 

[russ_watters]

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?

 

[R Power]

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!