# The expelling of fuel from a rocket (Momentum)

1. Oct 20, 2008

### tyro008

1. The problem statement, all variables and given/known data
A 4.00kg model rocket is launched, expelling 63.0g of burned fuel from its exhaust at a speed of 565m/s. What is the velocity of the rocket after the fuel has burned. Hint: Ignore the external forces of gravity and air resistance.

2. Relevant equations
Conservation of Momentum?

3. The attempt at a solution
I tried using the equation for the conservation of momentum by doing M1Vi=M-0.063Vf... but that did not work. I am stuck.

2. Oct 20, 2008

### tiny-tim

Hi tyro008!

I think you've assumed that 565 m/s is the fuel's actual speed … but the question probably means the speed relative to the rocket.

3. Oct 20, 2008

### tyro008

Do you mean that this is the rocket's speed?

4. Oct 20, 2008

### tiny-tim

No … the rocket starts at speed 0, and so the fuel is at speed 565.

But later the rocket is at speed v, say, and the fuel is at speed (565 - v).

5. Oct 20, 2008

### tyro008

so i did
(rocketmass X 0m/s) + (fuelmass X 565 m/s) = (rocketmass X v) + (fuelmass X (565 - v)
would this be right??

6. Oct 20, 2008

### tyro008

(this would be to find v)

7. Oct 20, 2008