# Homework Help: Momentum question -- A boy drops a box while sliding on ice...

1. Mar 22, 2016

### kellyxx

1. The problem statement, all variables and given/known data
A boy is holding a box and moving at a constant velocity of 2m/s on ice. The masses of the boy and the box are 40kg and 2kg respectively.Then the boy drops the box. What is the momentum of the boy just after dropping the box? Assume that the friction between the boy and the ice is negligible.
2. The attempt at a solution
I've tried to apply the conservation of momentum (m1u1+m2u2=m1v1+m2v2)
(40+2)(2)=2(0)+40v then v=84 kgms-1, but the correct answer should be 80kgms-1

2. Mar 22, 2016

### drvrm

after dropping the box -what is the velocity of the boy and the box ?

3. Mar 22, 2016

### Drakkith

Staff Emeritus
Notice what the question is asking for:

What did you find the momentum to in your work?

4. Mar 22, 2016

### kellyxx

The question didn't mention about that, but from my understanding, the velocity of the box after the drop should be 0ms-1

5. Mar 22, 2016

### Drakkith

Staff Emeritus
The velocity of the box before being dropped was 2 m/s. After being dropped, was there any acceleration in the horizontal direction to change its velocity?

6. Mar 22, 2016

### kellyxx

hmmm I guess there isn't any.

7. Mar 23, 2016

### haruspex

Just to be accurate about this, what happens after letting go the box is irrelevant to the boy's momentum. The question that needs asking is what horizontal force, if any, caused the box to go from 2m/s to 0m/s as the boy released it.

8. Mar 23, 2016

### The Vinh

Madame, I don't really think that velocity of the box is zero. Imagine that you are holding a lipstick, you go through a hole but you accidentally drop your favorite lipstick. Thanks to physics, it has your velocity instead of zero. Your lipstick survives ! If you release an object while you are having velocity, that object will have the same velocity as you.

9. Mar 23, 2016

### The Vinh

I think that in this circumstance, we don't need another horizontal force because OP didn't mention about it

10. Mar 23, 2016

### haruspex

You seem to misunderstand the point that Drakkith and I are making. Yes, we know there is no horizontal force. Our point is that in the absence of a horizontal force the box's velocity cannot change.

11. Mar 23, 2016

### The Vinh

Oh my god, I remembered writing three replies and now two left

12. Mar 23, 2016

### Drakkith

Staff Emeritus
I had to remove it since it contained the answer to the question. My apologies, I should have put that in the message I sent.

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