# Strange strange question

1. Jun 12, 2010

### superaznnerd

1. The problem statement, all variables and given/known data
a ball of 2kg falls from rest froma 5m height and collides elastically with the floor below. what is the magnitude change in the object's momentum?

2. Relevant equations

3. The attempt at a solution
I did S=So+.5aT^2
I solved for the time it takes to reach the ground, which is 1 second.
Next, I figured out that the velocity on impact with ground is 10m/s
I did 10m/s *2kg = 20kgm/s

however, the answer is 40kgm/s. Can someone explain? thanks

2. Jun 12, 2010

### rock.freak667

You have the initial momentum, you need to get the velocity after impact to get the final momentum.

3. Jun 12, 2010

### inky

But problem asked the change in momentum.

Impulse=change in momentum=Ft=mg*∆t

4. Jun 12, 2010

### Redbelly98

Staff Emeritus
They want the change in momentum during the impact with the floor, not during the initial drop.

5. Jun 13, 2010

### Staff: Mentor

Think what "elastically" means.

6. Jun 13, 2010

### inky

I think elastically means after collision the ball moves up. If we consider this, we need coefficient of restitution.

7. Jun 13, 2010

### Redbelly98

Staff Emeritus
The coefficient of restitution = 1 for an elastic collision.

Please let the OP solve the problem now.

8. Jun 13, 2010

### inky

e=1 for perfectly elastic collision. I mean before collisin and after collision speed are the same for e=1. Actual case is not perfectly elastic.

9. Jun 13, 2010

### Redbelly98

Staff Emeritus
Disagreed. From Giancoli's book Physics (5th ed.): "a collision, in which total kinetic energy is conserved, is called an elastic collision". And reading further: "collisions in which kinetic energy is not conserved are said to be inelastic collisions", and "if two objects stick together as a result of a collision, the collision is said to be completely inelastic."

10. Jun 13, 2010

### superaznnerd

now I get the answer is 0. right before the collision it is 20kgm/s down. right after it is 20kgm/s up. Thus, there is no change in magnitude of momentum...

rebelly: can you explain how the stuff you wrote is relevant??

11. Jun 13, 2010

### Staff: Mentor

Hm, could be my English fails me here, but looks like you are right. Momentum magnitude doesn't change, even if momentum changes by 40 kgm/s. But I can be wrong.

What you posted is a direct quote of the question wording, or was it translated?

You can safely ignore discussion with inky.

12. Jun 13, 2010

### Redbelly98

Staff Emeritus
But there is a change in the direction of momentum. Momentum is a vector, so there is a change in the momentum. Hint: express the upward momentum as positive, and downward as negative.
It was addressed to stuff inky posted, don't worry about it.

13. Jun 13, 2010

### Redbelly98

Staff Emeritus
Okay, I think I see the source of confusion here.

They are asking you to first find the change in momentum, and then get the magnitude of that change. It is not asking for the change in the magnitude of momentum.

14. Jun 13, 2010

### superaznnerd

that sounds right ill ask my teacher 2moro

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