Posted by **Cassy** on Wednesday, August 18, 2010 at 3:05pm.

During a lift off , the thrust a rocket produces must overcome the force of gravity in order to lift the rocket off the ground. If a rocket with a mass of 5kg produces a force of 180N, how fast will the rocket be travelling 3 seconds after ignition?

- physics -
**drwls**, Wednesday, August 18, 2010 at 3:46pm
First calculate the acceleration.

a = (Thrust-Weight)/M = (180-M*g)/M

180/5 -g = 26.2 m/s^2

Assume the mass remains the same (although it actually must decrease as propellant is released) and use

Y = (1/2) a t^2

to compute how far it rises (Y) in t=3 seconds.

- physics (P.S.) -
**drwls**, Wednesday, August 18, 2010 at 3:48pm
I just noticed they asked for the velocity after three seconds, not the distance.

Use V = a t for velocity.

- physics -
**Cassy**, Wednesday, August 18, 2010 at 4:37pm
thank you drwls

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