# Work done by rocket motors

1. May 17, 2012

### Taylor_1989

1. The problem statement, all variables and given/known data
Rocket motors make a rocket of mass of 2200kg accelerate at 2.5m/s^2 over a distance of 650m in deep space. What is the work done?

3. The attempt at a solution
F=m*a 2200kg*2.5m/s^2 = 5500N
5500*650= 3575000

When I look at the answer page it tells me the force causing acceleration is 11000N. I generally cant see where this figure has come from. The only thing I can see if i double the answer I get it = the answer that is given wd= 7150000.

The only other possible solution I can come to is that, the mass of the rocket decrease as it accelerates, so the original mass of rocket was 4400kg. If I used this figure it would give me the right answer. Is this the right train of thought?

2. May 17, 2012

### Staff: Mentor

my guess is the book is wrong, maybe someone changed the problem parameters for a new edition and forgot to change the answer like halving the acceleration or mass of the rocket in the problem and forgetting to halve it in the answer.

work = mass * acceleration * distance traveled

and they don't give you enough info about rocket engine burn rates to factor in a changing rocket mass. also typically in a problem saying the rocket is 2200kg means that's what it is initially.

3. May 17, 2012

### Taylor_1989

Thanks for the reply, I am starting to think this to due to other question in the book are not matching with the formulae e.g k.e of a cheetah of 48Kg running at 30m/s = 43200 which clearly is not right according to the formulae.