Homework Help: physics

Posted by beavis on Wednesday, February 15, 2012 at 5:04pm.

(ignore the other question i posted this is the right one)
A rocket has a pre-ignition mass of 1100kg. On ignition it burns fuel at rate of 20kg s-1 producing a thrust of 10000N. How long after ignition of the fuel, will the rocket lift off?
also explain how is it possible for a rocket to accelerate in space.
(i cant work it outt. i know it involves the rocket thrust equation some how, the newtons 2nd law and suvat's..) thank you

physics - drwls, Wednesday, February 15, 2012 at 5:31pm
The rocket weight is
M = (1100 - 20 t)*g = 10,780 - 196*t newtons

That equals the thrust when
196*t = 780 N
t = 3.98 seconds

Rockets acclerate during each stage because the thrust stays the same while the mass decreases (until burnout, when the fuel of that stage is gone).
physics - beavis, Wednesday, February 15, 2012 at 5:43pm
ahh thank you so much!
can you write the actual formulas that you've used because im not sure if i understood it correctly

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