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Pages

1Where is Mars?2How long will it take to get to Mars?3Getting to Mars4How do rockets get off the ground?5Producing Thrust6What happens when rocket fuel burns?7Your mission to Mars!8Our need for oxygen9How oxygen gets to our cells10The effects of gravity11How low gravity affects bones12Preventing bones and muscles from becoming weak13Living and working in space14What do astronomers know about Mars?15A space-craft view of Mars16Exploring Mars17Energy for exploration18The rocky surface of Mars19More about the surface of Mars20The Martian atmosphere21The importance of water22The water cycle on Earth23Is there water on Mars?24Surviving on Mars25Your Martian home26Farming on Mars27Soil on Mars28Plants for Mars29Getting energy on Mars30Where will you go next?

Practicals

1Modelling the solar system in the school yard2Rocket Practical3Investigate the Energy in Fuels4Making a Telescope5What factors affect the output of a solar cell?6Modelling Diffusion7Examining Slides of Blood8What bones are like without calcium salts9Flame Test to Identify Metals10The Different States of Water11Water Purification for Human Consumption12Factors Affecting Germination13Solar Cells for Devices14Mars Bingo
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Everything on the surface of the Earth is pulled down towards its centre.

An image depicting a falling object. Did you know 1 falling apple's force is equal to 1 Newton?

Questions

1
What is the name of the force that exerts this pull?
2
What is the approximate value of acceleration due to this force?
m/s2

The force of gravity gives the rocket (and everything else on earth!) its weight. If there is no force of gravity, an object is said to be 'weightless' but still has mass (ie. the material it is made of). A rocket must produce a very strong 'thrust' force to act against its weight so that it can push (propel) itself off the ground at 'lift off'.

Try working out the answers to the following questions before clicking on the 'Show Answer' buttons – how many did you get right?

1
In which direction does the force of gravity act?
Towards the Earth
2
In which direction does the thrust force act?
Away from the Earth

These two forces (thrust and weight) act in opposite directions, one upwards (thrust), and the other downwards, towards the Earth (weight). This means that they act against each other. The actual force which lifts the rocket off the ground is called the 'resultant' force - this is the difference between the forces of weight and thrust. You can find the value of this by subtracting the rocket's weight from its thrust, as shown in the simple formula below:

Resultant force (R) = Thrust Force (T) − Weight (W)
A diagram explaining resultant force

Try working out the answers to the following questions before clicking on the 'Show Answer' buttons – how many did you get right?

1
If the weight of a rocket is 500,000N, and thrust is 800,000N, will the rocket get off the ground?

Yes, because the thrust is bigger than the weight

2
If the weight of a rocket is 500,000N, and thrust is 800,000N, what would be the resultant force acting on this rocket to get it off the ground?

While Resultant Force = Thrust - Weight,

	R = T − W
	R = 800,000N − 500,000N
	R = 300,000N

So the resultant force will be 300,000N

3
What other force(s) might act on the rocket as it is launched?

friction from gases in Earth's atmosphere

Rocket Practical

Send a bottle to the moon!

View Practical >