# Homework Help: Very basic, short impulse problem. Just need some explanation.

1. Oct 22, 2012

### AATroop

1. The problem statement, all variables and given/known data
If the magnitude of the Earth's momentum is P, what is the magnitude of the impulse that the sun applies to the Earth during six months?

2. Relevant equations

3. The attempt at a solution
I know the solution is 2P, and I'm pretty sure it's just due to the change in direction that would occur when looking at the different time steps; when t =0, the Earth (we can assume) is moving in the positive x direction, and at t= 6 months, the Earth is moving in the negative x direction, so | m*(-v) - m*v | = 2mv or 2P. Is this adequate?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 22, 2012

### Staff: Mentor

Sounds like you understand it just fine. The key physics is understanding that impulse = Δmomentum. And that the momentum of the earth changes from P to -P over six months.

3. Oct 22, 2012

### AATroop

OK, thanks. Just wanted to be sure, because I often confuse |ΔP| with Δ|P|.