Momentum of two carts

1. Apr 5, 2015

Westin

1. The problem statement, all variables and given/known data

Two identical carts A and B roll down a hill and collide as shown in the figure below.

i) A starts from rest. It rolls down and collides head-on with B which is initially at rest on the ground. The two carts stick together.

ii) A and B are at rest on opposite sides of the hill. They roll down, collide head-on and stick together.

Which statement is true about the two-cart system just after the carts collide in the two cases?

Select all that are True.
The momentum of the system is zero in case (ii).
The momentum of the system is the same in both cases (but no zero).
The momentum of the system is greater in case (ii) than in case (i).
The momentum of the system is greater in case (i) than in case (ii).

2. Relevant equations
Conservation of Momentum

3. The attempt at a solution

I believe the answer is #1 and #4 are true. The momentum of the system is greater in case (i) than in case (ii) because case (i) is still moving unlike case (ii).

I believe the momentum is zero in case (ii) because it started from rest ad ended at rest

File size:
55.1 KB
Views:
107
2. Apr 5, 2015

ehild

It is correct in principle, but not a "case" is moving, and not the "momentum" is in rest. The momentum can have some value, zero or not zero.

3. Apr 5, 2015

Westin

So you're saying we don't know if the momentum is zero or nonzero so I'm wrong?

4. Apr 5, 2015

ehild

You know if the final momentum of the whole system is zero or not zero. The answer "#1 and #4 are true" is correct. What is wrong is "because case (i) is still moving unlike case (ii)". Use proper subject in the sentence.

Last edited: Apr 5, 2015
5. Apr 5, 2015

Westin

Case (i) is non zero and Case (ii) is zero ?

6. Apr 5, 2015

ehild

What is zero? A case has no numerical value.