# The Rocket

Another problem using resultant forces.
A rocket with a mass of 20 000 kg uses a take off engine with a thrust of 250 000 N.
(a) Calculate the initial acceleration of the rocket
(b) what happens to the acceleration of the rocket as it rises into the air?
(Take 9 = 9.8 ms-2)

(a) Resultant force producing acceleration = Thrust - Weight = 250 000 - 196 000 = 54 000 N
Initial acceleration = F/m = 54 000/20 000 = 2.7 ms-2.

Notice that it is the RESULTANT force that has been used to calculate the acceleration of the rocket.

(b) As the rocket rises the mass with decrease as the fuel is used up and if we assume a constant thrust then the acceleration will increase. There is also a small reduction in air friction as the rocket climbs although this might be balanced by increased drag as the speed increases. The change in the pull of gravity will only be significant after climbing hundreds of kilometres.

To see a video of a 'tea bag' rocket please click on the link.

The tea bag

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB

© Keith Gibbs 2020
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