A rocket needs a force to get it off the
ground – we call this force the thrust of the rocket. The
heavier the rocket the bigger the thrust needs to be to get the rocket to take off.
To
just lift the rocket off the launch pad the thrust needs to balance the downward pull of the
earth on the rocket. In other words the thrust must be equal to the weight of the rocket.
To get the rocket to accelerate upwards the thrust needs to build up so that it is
bigger then the weight of the rocket. You may have noticed this if you have seen pictures of
the US Space Shuttle or other large rockets taking off. It takes a few seconds before they
really begin to accelerate.
The thrust of a firework rocket builds up very quickly and
so they accelerate upwards almost as soon as the fuel is burning.
As the rocket travels higher and higher above the Earth if uses fuel and so gets lighter – this means that the thrust does not need to be so big to accelerate the rocket. Also the higher it gets the smaller the gravity pull of the Earth becomes although this does not really make a big difference until the rocket is many hundreds of kilometres up.