(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Hi, our teacher set this in class. I think its above what we have to know for the syllabus and have tryed to work it out but i dont think im doing it right. I just want a few pointers, not the answer!

ok...

A rocket with mass 5000kg of which 4000kg is propellant is launched vertically. The fuel is consumed at a rate of 50kg per second. What is the least velocity of the exhaust gases if it is to release the rocket immediately after firing.

2. Relevant equations

F=ma

Suvats

Momentum=mv

F=Change in mv/t

3. The attempt at a solution

Ok i experimented a bit with it but had too many unknowns to really make much of it. I saw you could model the rocket and gas given off as 2 different particles acting in opposite directions. The momentum of the rocket equaling the momentum of the gas given off. This would mean the velocity of the gas cloud would be much greater than the velocity of the rocket at the 1 second mark.

I multiplied the mass of the rocket (and propellant) after 1 second by g to get 48510. I figured this is the weight of the rocket which acts downwards. The force provided by the gas cloud pushing the rocket upwards must exceed the weight of the rocket downwards for the rocket to move up. The mass of gas released is 50kg so i divided the minimum force needed by the amount of gas to get the acceleration which was 970.2.

Im pretty sure the working ive done is wrong as i haven't used the formula for momentum. I tryed expressing the velocity of the gas with the velocity of the rocket in the momentum formula and played around with substituting in stuff but havnt got anywhere.

I dont think the working can be too complicated, i just think there is something im missing. Any hints or pointers would be appreciated to get me on the right track. Thanks

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# Homework Help: Momentum with changing mass

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