# Conservation of Momentum and Collisions

1. Apr 19, 2015

### Charles W

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

A lorry of mass 20,000kg is travelling at 20.0 metres per second. A car of mass 900kg is travelling at 30.0 metres per second towards the lorry.

What is the magnitude of the total momentum?

1. 209 kNs
2. 373 kNs
3. 427 kNs
4. 1045 kNs

2. Relevant equations

Linear Momentum = Mass * Velocity

3. The attempt at a solution

I have tried to find the momentum of each object:

20, 000 * 20 = 40, 000 (for the lorry)
900 * 30 = 27, 000 (for the car)

UPDATE

Can't believe I made such a stupid mistake:

20, 000 * 20 = 400,000

Therefore: 400, 000 - 27, 000 = 373, 000 (Option 2)

Last edited: Apr 19, 2015
2. Apr 19, 2015

### Vatsal Sanjay

Momentum is a vector quantity.

3. Apr 19, 2015

### Charles W

I am aware of that, but how does it help to solve this problem?

4. Apr 19, 2015

### Orodruin

Staff Emeritus

5. Apr 19, 2015

### Charles W

That would probably be it - I can't believe I made such a stupid mistake. I'll edit my original question

6. Apr 19, 2015

### Vatsal Sanjay

I think you missed one zero
By the way my apologies, I didn't see you used a minus sign.

7. Apr 19, 2015

### Charles W

No worries! Thanks for your help :)

8. Apr 19, 2015

### Orodruin

Staff Emeritus
Also note that, as stated, there is not one definite answer. It depends on whether the car is in front of or behind the lorry, since these would imply different directions for "towards the lorry".