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

1. Jun 1, 2010

### 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?

2. Jun 1, 2010

### novop

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

3. Jun 1, 2010

### 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.

4. Jun 1, 2010

### 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

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5. Jun 1, 2010

### 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.

6. Jun 1, 2010

### espen180

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.

7. Jun 2, 2010

### R Power

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

8. Jun 2, 2010

### xxChrisxx

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

9. Jun 2, 2010

### R Power

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

10. Jun 2, 2010

### xxChrisxx

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, thats 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.

11. Jun 2, 2010

### R Power

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!

12. Jun 2, 2010

### Staff: Mentor

But then you say...
Two very different statements.

13. Jun 2, 2010

### 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??????

14. Jun 2, 2010

### xxChrisxx

EDIT: nm you realise 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.

Last edited: Jun 2, 2010
15. Jun 2, 2010

### Staff: Mentor

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.

16. Jun 2, 2010

### Staff: Mentor

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?

17. Jun 2, 2010

### R Power

ok i think i got it!

18. Jun 2, 2010

### R Power

that's how bodies interact and forces are applied

19. Jun 2, 2010

### 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?

20. Jun 2, 2010

### xxChrisxx

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.

Last edited: Jun 2, 2010