Kg m/s Change in Momentum of Trolley

[dh743]

Homework Statement

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?

Homework Equations

p=mv
$$\Delta$$p=p final - p initial

The Attempt at a Solution

$$\Delta$$p=p final - p initial
=7.8 - 4.8
=3

 

[substance90]

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

 

[Doc Al]

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?

 

[dh743]

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

the answer I've got is 1.8

 

[dh743]

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?

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.

 

[Doc Al]

Thanks for your help but i ended up with the correct answer but negative (-1.8). Would this be correct in this situation?

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

I would have thought an increase in mass (the object) would mean an increase in momentum.

Does the cart speed up or slow down?