# Finding time with force, mass and velocity

1. Nov 22, 2012

### Ally385

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

Small rockets are used to make tiny adjustments in the speeds of satellites. One such rocket has a thrust of 35 N. If it is fired to change the velocity of a 71700 kg spacecraft by 66 cm/s, how long should it be fired?

2. Relevant equations

p = mv
Impulse = FΔt

3. The attempt at a solution

Well I think that you might have to find momentum and then plug that into another equation. I'm just not sure what equation that would be since the impulse = FΔt doesn't use momentum.

2. Nov 22, 2012

### Staff: Mentor

Look up the 'impulse-momentum' theorem.

Start here: The Impulse-Momentum Change Theorem

3. Nov 22, 2012

### Ally385

So that would make it:

F Δt = Δ p

So that makes that

Δt = Δp/ F
Δt = 4732200 - 0 / 35
Δt = 135205.7

But now how do I get rid of the delta?

4. Nov 22, 2012

### Staff: Mentor

Right.

Careful with units. Convert the speed to standard units of m/s.

Δt is what you are solving for. (You don't need to get rid of the delta; Δt is the time that the thrust must be applied.)

5. Nov 22, 2012

### Ally385

So is it in cm/s now because that's what it says in the problem? And then do I have to change it to minutes because that's what it asks for next to the answer blank. (Sorry forgot to put that in.)

6. Nov 22, 2012

### Staff: Mentor

Yes. You need to convert that to m/s.
Exactly. The equation will give you the time in standard units of seconds. If you need the answer in minutes, you'll have to convert units.

7. Nov 22, 2012

### Ally385

Awesome! Thanks :)