# Please verify this notion regarding . momentum

1. Apr 12, 2013

### stephenn

Please verify this notion regarding..... momentum

Hello. Could someone verify this notion about momentum.... thanks

here is info:
- The conserved closed system here, is a vehicle and the planet (via the road attached)
- A stationary vehicle has no momentum
- A moving vehicle has some momentum

here is point to verify:
A moving vehicle has created a quantity of momentum (forward direction for vehicle) AND it has created a quantity of momentum (opposite direction for planet)
* So... the total quantity of momentum is exactly half each?
As p=mv
* So... the vehicle will have very small m and large v quantity
* So... the planet will have large m and very small v quantity

I wish to know if I can correctly state...... the total quantity of momentum is half each (for vehicle and planet)

[this is linear momentum and no other factors effect the closed system]

much appreciate thoughts, cheers stephenn

2. Apr 12, 2013

### Staff: Mentor

You've pretty much on the right track here, but it is not correct to say that the total amount of momentum is "half each". Momentum is a signed quantity (that is, direction matters) so you should say that the total momentum is still zero:

Vehicle's momentum we'll call P; we get this by multiplying a small mass times a large velocity. Now the earth's momentum is -P; we get this by multiplying an enormous mass times a very small negative velocity. Total momentum is the momentum of the earth plus the momentum of the vehicle, P+(-P) = 0.

Doing it this way, the total momentum is conserved: zero before and zero after.

3. Apr 12, 2013

### stephenn

Thanks for reply.... and workings.... I understand that the phrase of half and half doesn't carry with it, the direction factor (one being opposite to the other).

I still wish to test "worded" / "spoken" notions, that will hold true:
so for the same notion:

In the closed system, mentioned..... should this hold true?:
Any momentum present (for vehicle and planet), will be identical in quantity, but apply in opposite directions.

4. Apr 12, 2013

### Staff: Mentor

Better to say: The change in momentum of each will be equal and opposite.

5. Apr 12, 2013

### stephenn

haha, I was prodding and poking at my sentence to get it more concise.... got the equal and opposite bit.... your "change" and neat phrasing gets a thumbs up, thankyou.