# Homework Help: Collision and Impulse, Linear Momentum

1. Jul 18, 2006

### kappcity06

can someone help? I have no isea how to do this problem.

A force in the negative direction of an x axis is applied for 21 ms to a 0.40 kg ball initially moving at 10 m/s in the positive direction of the axis. The force varies in magnitude, and the impulse has magnitude 32.4 Ns.

(a) What is the ball's speed just after the force is applied? m/s

(b) What is the average magnitude of the force? N

2. Jul 18, 2006

### Staff: Mentor

Tell us what you know about impulse, momentum, and the impulse-momentum theorem.

3. Jul 18, 2006

### kappcity06

i know about elastic and inelastic collsions

4. Jul 18, 2006

### kappcity06

i know the formula for those

5. Jul 18, 2006

### kappcity06

i know p=mv

6. Jul 18, 2006

### Staff: Mentor

They gave you the impulse. What does that tell you?

7. Jul 18, 2006

### kappcity06

p1i+p2i=p1f+p2f

8. Jul 18, 2006

### kappcity06

i should use p=mv

but thsat would give be AN INTIAL SPEED OF 81

32.4=.4v

9. Jul 18, 2006

### Staff: Mentor

impulse-momentum theorem

Impulse = $\vec{F}_{ave} \Delta t = \Delta \vec{p} = m \Delta \vec{v}$

10. Jul 18, 2006

### kappcity06

that gives me 91

32.4=.4(v-10)

11. Jul 18, 2006

### Staff: Mentor

What's the direction (and thus sign) of the impulse compared to the direction of the initial velocity?

12. Jul 18, 2006

### kappcity06

negative to the x axis then postivite the force is postive

13. Jul 18, 2006

### Staff: Mentor

What's the direction of the initial velocity?
What's the direction of the impulse?
What's the direction of the change in velocity?
What's the direction of the final velocity?

14. Jul 18, 2006

### kappcity06

the direction of the intial volcity is positve.
the direction of the impulse is negative
the direction of the change in velocity is negative

the direction of the final velocity is negative

15. Jul 18, 2006

### Staff: Mentor

Good. So you should have no problems calculating the final speed.

16. Jul 18, 2006

### kappcity06

ok thank you

17. Jul 20, 2006

### thiotimoline

After my analysis, first get the initial momentum of the ball in the positive x-direction. Second, get the force applied on the ball in the negative x-direction. Impulse is mass x change in velocity. Problem is whether the total force acting on the ball is sufficient to change the direction of ball.