# Mechanics Question of two balls

1. Apr 20, 2006

### turnstile

Mechanics Question :)

Hi.

Two small balls A and B have masses 0.5kg and 0.2 kg respectively. They are moving towards each other in opposite directions on a smooth horizontal table when they collide directly. Immediately before the collission, the speed of A is 3m/s and the speed of B is 2 m/s. The speed of A immediately after the collision is 1.5 m/s. The direction of motion of A is unchanged as a result of the collision.

By modelling the balls as particles, Find;
(a) the speed of B immediately after the collision.

(b) the magnitude of the impulse exerted on B in the collision.

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For part (a) i am getting 2.3 m/s.. however my teacher's worked solution is giving 1.75 m/s.... now.. i dont know which one is correct...

for part B... i use Ft=mv-mu and get -0.75 ... i think im wrong....

Thanks
Faraz

2. Apr 20, 2006

### Staff: Mentor

Show how you solved it and we can take a look.

3. Apr 20, 2006

### J77

conservation of momentum

0.5*3-0.2*2=0.5*1.5+0.2*X

where X is the unknown speed

Last edited: Apr 20, 2006
4. Apr 20, 2006

### turnstile

HI there..
why is it 0.5 x3 MINUS 0.2*2 ?
because my 2.3 comes from adding these two....

5. Apr 20, 2006

### J77

They're travelling in opposite directions initially...

6. Apr 20, 2006

### turnstile

ah.. thanks a lot J77... clear now.

im quite sure my -0.75 is wrong

7. Apr 20, 2006

### J77

Well, impulse = change in momentum...

momentum before = 0.2 * 2

momentum after = 0.2 * 1.75

implies impulse = 0.2 * (2 - 1.75) = 0.2 * 0.25 = 0.05

8. Apr 20, 2006

### Staff: Mentor

Why do you have a minus sign? What direction is the impulse on B?

Also, regardless of direction, they asked for the magnitude of the impulse.

9. Apr 20, 2006

### turnstile

I= 0.2 x (2+1.75)= 0.75Ns

I just looked at the mark scheme and it confirms that....

10. Apr 20, 2006

Momentum is a vector, so you must consider the direction. Based on your working for (a), B moves in the positive direction after the collision, and in the negative direction before the collision.

Do you now understand how the correct answer of 0.75 Ns is obtained?

P/S Impulse on B = Final momentum of B - Initial momentum of B

11. Apr 20, 2006

### turnstile

Thanks a lot... i get it now...
:)

12. Apr 20, 2006

### Staff: Mentor

PS: J77 made an error in post #7--he had the wrong sign for the initial momentum. (This is the same error that he pointed out to you in your solution to part a.)

13. Apr 21, 2006

oops!