# Homework Help: Force on rocket

1. Oct 31, 2012

### songoku

1. The problem statement, all variables and given/known data
A rocket with mass 60 x 103 kg (25 % of it is "empty mass"). The rocket burns the fuel at rate 250 kg/s and expel the gas at relative speed 1.6 km/s. Find the force experienced by the rocket and the time until the force equals to zero

2. Relevant equations
Δp = F.Δt
p = mv
W = mg

3. The attempt at a solution
I am not sure what is the meaning of "empty mass". Is it the mass of rocket without fuel?

This is my attempt:
Force experienced by rocket
= Δp / Δt
= Δ(mv) / Δt
= v . Δm / Δt
= 1.6 x 103 x 250
= 4 x 105 N

Is that correct?

For the time until the force ceases:
Assume empty mass means the mass of rocket without fuel, so the mass of fuel = 75 % of 60 x 103 kg = 45,000 kg

Time = 45,000 / 250 = 180 s ---> this means that the rocket accelerates for only 3 minutes???

Thanks

2. Oct 31, 2012

### haruspex

That all looks right to me, except that it's not clear whether "the force experienced by the rocket" is supposed to be just that due to the engine or is to include gravity.

3. Nov 2, 2012

### songoku

If includes weight, it should be 4 x 105 N - weight, correct?

4. Nov 2, 2012

Yes.

5. Nov 3, 2012

### songoku

OK thanks for your help :)