Finding time with force, mass and velocity

[Ally385]

Homework Statement

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?

Homework Equations

p = mv
Impulse = FΔt

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.

 

[Doc Al]

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.

Look up the 'impulse-momentum' theorem.

Start here: The Impulse-Momentum Change Theorem

 

[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?

 

[Doc Al]

So that would make it:

F Δt = Δ p

Right.

So that makes that

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

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

But now how do I get rid of the delta?

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.)

 

[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.)

 

[Doc Al]

So is it in cm/s now because that's what it says in the problem?

Yes. You need to convert that to m/s.

And then do I have to change it to minutes because that's what it asks for next to the answer blank.

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.

 

[Ally385]

Awesome! Thanks :)