# Homework Help: Change in momentum

1. May 16, 2010

### dh743

1. The problem statement, all variables and given/known data
A 2.4kg cart is moving with a constant speed of 2m/s along a smooth horizontal plane when a 1.5kg object is dropped vertically onto it. What is the change in momentum of the trolley?

2. Relevant equations
p=mv
$$\Delta$$p=p final - p initial

3. The attempt at a solution
$$\Delta$$p=p final - p initial
=7.8 - 4.8
=3

2. May 16, 2010

### substance90

So what is the problem with this one? Seems perfectly ok to me.

3. May 16, 2010

### Staff: Mentor

You need to find the momentum of the cart before and after the object drops onto it. You have the cart's speed before the collision, so you can find its initial momentum. But what is its speed after the collision?

Hint: What's conserved during any collision?

4. May 16, 2010

### dh743

the answer i've got is 1.8

5. May 16, 2010

### dh743

Thanks for your help but i ended up with the correct answer but negative (-1.8). Would this be correct in this situation? I would have thought an increase in mass (the object) would mean an increase in momentum.

6. May 16, 2010

### Staff: Mentor

That happens to be correct, but how did you arrive at this answer?

Does the cart speed up or slow down?