The expelling of fuel from a rocket (Momentum)

[tyro008]

Homework Statement

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.

Homework Equations

Conservation of Momentum?

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.

 

[tiny-tim]

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.
…
I tried using the equation for the conservation of momentum by doing M1Vi=M-0.063Vf... but that did not work. I am stuck.

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.

 

[tyro008]

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

 

[tiny-tim]

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

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).

 

[tyro008]

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

 

[tyro008]

(this would be to find v)

 

[Ashleyz]

answer - 8.89m/s...Is this right?

 

[tyro008]

hey yeah it is! thanks so much to all you guys for helping me, i REALLY appreciate it! :]