# Homework Help: Physics momentum problem

1. Aug 14, 2010

### pokeefer

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

Small rockets are used to make small adjustments in the speed of satellites. One such rocket has a thrust of 35 N. If it is fired to change the velocity of a 72 000 kg spacecraft by 63 cm/s, how long should it be fired?

2. Relevant equations

Momentum = Mass x Velocity

3. The attempt at a solution

After reading the instructions, I'm still having difficulty on this particular problem.

There is a formala called pbefore = pafter

Where momentum in one object transfers to another when hit. But I just don't know where to begin.

Thanks.

2. Aug 14, 2010

### Staff: Mentor

Hint: Consider the impulse delivered by the force. (Look it up!)

3. Aug 14, 2010

### pokeefer

Thanks but I don't know what that has to do with my problem. Can you post an explanation of the formula or the solution?

4. Aug 14, 2010

### Staff: Mentor

What's the definition of impulse and what does it have to do with momentum? Look it up!

5. Aug 14, 2010

### pokeefer

How am I supposed to calculate impulse with the information given?

6. Aug 15, 2010

### Staff: Mentor

What's the definition of impulse?

7. Aug 15, 2010

### pokeefer

Iv'e gotten this solution:

F = (mass x velocity) / time
35N = 72,000 x 0.63 / t
35N = 45,360 kg m/s / t
35t = 45,360kg m/s
T = 1296 seconds

Is it right?

8. Aug 15, 2010

### Staff: Mentor

Yes, very good.

Impulse = FΔt

And impulse equals the change in momentum, Δ(mv).

So:
FΔt = Δ(mv) = mΔv
Δt = mΔv/F