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

[R Power]

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?

 

[novop]

The second ball is just moving perpendicularly to the point of contact an any case.

 

[espen180]

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.

 

[R Power]

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

 

[xxChrisxx]

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.

 

[espen180]

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

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.

 

[R Power]

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.

Why do you think of parallel force component ? Force is applied in vertical direction and so should the red ball move in vertical direction.

 

[xxChrisxx]

Why do you think of parallel force component ? Force is applied in vertical direction and so should the red ball move in vertical direction.

Why do you think the force would act in the direction of movement?

 

[R Power]

Why do you think the force would act in the direction of movement?

When did i say this? I said that movement should occur in the direction of force applied.

 

[xxChrisxx]

When did i say this? I said that movement should occur in the direction of force applied.

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, that's 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.

 

[R Power]

To me that means you think that the force applied to the second ball is in the direction of the 1st balls motoin.

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!

 

[Doc Al]

When did i say this? I said that movement should occur in the direction of force applied.

But then you say...

Black ball should apply force in the direction it is moving!

Two very different statements.

 

[R Power]

doc
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?

 

[xxChrisxx]

EDIT: nm you realize 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.

 

[Doc Al]

doc
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?

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.

 

[Doc Al]

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?

 

[R Power]

ok i think i got it!

 

[R Power]

that's how bodies interact and forces are applied

 

[DaveC426913]

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?

 

[xxChrisxx]

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?

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.

 

[R Power]

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.

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?

 

[R Power]

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?

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!

 

[xxChrisxx]

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?

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.

 

[R Power]

so what about the answer i think in post 22?

 

[vlado_skopsko]

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 parallel to the contact surface.

 

[DaveC426913]

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 parallel to the contact surface.

Oh I see what you're saying. In order for the moving ball to impart a direction on the stationary ball, there would have to be an area of contact, and some friction. With two rigid balls and an elastic colllision, the contact surface between the two balls is effectively a point.

So, in laypersons terms, the stationary ball is "blind" to anything other than the point of contact, which has neither significant area, nor direction. Without either the those things, the force acting on it is simply along a line from contact point to centres of mass (i.e. the normal).

 

[R Power]

what about my explanation in post 22

 

[Red_CCF]

EDIT: nm you realize 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.

I actually have a question about normal forces. I think this may be what the OP is confused about as well, as to why the normal force acts perpendicular. How is it proven that normal force is acts perpendicular to the surface/tangent of the contact? What kinds of experiments can be done to prove that this occurs for all contact between two objects?

Secondly, if an object is moving then the normal force should not be constant right? How would I calculate this normal force, and if the black ball was moving at constant velocity, would it be the momentum transfer that causes this normal force to exist? What information is necessary to graph the Force - Time graph of this interaction (to find the impulse transmitted)? Is it triangular (ignoring friction between the ball and of the ground)?

 

[DaveC426913]

I actually have a question about normal forces. I think this may be what the OP is confused about as well, as to why the normal force acts perpendicular.

Well, by defintion for starters. Normal means "perpendicular".

So, the question is more accurately:

Why is the force exerted as a normal force?

 

[Doc Al]

So, the question is more accurately:

Why is the force exerted as a normal force?

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.

 

[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!