Science at key stage 3    (Year 9)

• Remind pupils of the distinction between mass and weight and that weight is the force due to gravity. Present pupils with a picture of the Earth and ask them to indicate what would happen to a ball dropped at different positions. Establish that the ball would fall towards the Earth and that this is the definition of 'downwards'. Generalise that gravity is an attractive force between any masses, but that we feel its pull only with very large masses, such as the Earth.
• Use the historical association of the newton to highlight that the concept of gravity was a very difficult one for scientists to understand. Explain that while Newton did not 'discover' gravity, he formulated the concept mathematically and realised its importance, at levels ranging from apples falling from trees to the orbits of planets. Drop an apple into pupils' hands to feel the gravitational force of 1 newton. Ask pupils to lift masses of a known value and feel the attractive force of the Earth's gravity pulling them back.

• Pupils may have been introduced to the idea that they would weigh less on the surface of the Moon in unit 7K 'Forces and their effects'. Remind them of this using video images of lunar exploration. Ask them to explain why they think this is, ensuring that they recognise that mass is unchanged. Encourage inference, eg The gravity is less on the Moon. Using questions, eg Why should it be less?, helps pupils to relate the decrease to the lesser mass of the Moon.
• Tell pupils that the Moon's gravitational pull is about one sixth that of the Earth's and ask them to calculate the weight of everyday objects if taken to the Moon. Ask them to imagine how this might affect daily life on the Moon, eg What things would be easier to do? What would be more difficult? Extend by providing data on the gravitational pull on different planets in the solar system. Reinforce this by showing pupils a number of cereal packs or bags of sugar, which look the same but are labelled with the weight they would be on each of the planets. Label each with the planetary name, and ask pupils to handle them to get an idea of the differences.

• Ask pupils to think about how rockets get away from the Earth. Illustrate using secondary sources, eg using software simulations or video clips of space shuttles, or demonstrate with a water rocket.
• Establish that, for a rocket to get off the ground, a thrust force greater than the rocket's weight is needed. It is clear that it gets easier the higher the rocket travels (less fuel is needed). However, the rocket has less mass, having shed some of its load, so it would be accelerating even if gravity were the same.
• Ask pupils to use secondary sources to present an account of space exploration.

• Provide pictures of stages in a mission to the Moon. Ask pupils to sequence these and to write about how gravity is influencing the journey at different points, eg at take-off, in orbit, leaving orbit, halfway to the Moon, etc.
• Extend by providing cards with possible weight and mass of astronauts, and ask pupils to write captions about how it would feel at different stages.

• Present, using secondary sources, some alternative models of the solar system, eg the ideas of the ancient civilisations of Egypt, India, Greece, and the contributions of Thales, Aristarchus of Samos, Copernicus. Ask pupils to consider a model and to use secondary sources, eg internet sites, encyclopedias, to suggest the evidence for and against it. Pupils could be asked to participate in a class debate defending a particular model, eg that the Earth is the centre of the universe.
• With the class, summarise the main strengths and weaknesses of each model and ask pupils to write up their own conclusion.

• Show pupils a video on the formation of the solar system. Ask them to extract information about the effects of gravity on the resultant shapes and motions of the planets and other bodies. Discuss the main points with pupils.
• Help pupils to make the link between circular motion and inward force by swinging a rubber bung on a string in a circular motion. Point out the tension in the string, which suggests an outward pull by the bung. Help them to realise that the bung's circular motion can only be maintained by an inward force, which is exerted by the string. Ask pupils what would happen if the string were cut. This is a model of the gravitational pull exerted on the planets by the Sun.

• Relate the orbit of the Moon round the Earth to that of the planets round the Sun. Explain that since the Moon is so close to the Earth, its orbit is influenced by the Earth's gravity rather than that of the Sun.
• Help pupils to visualise how an asteroid could be captured into an orbit by the gravitational field of a planet, eg using a software simulation.

• Ask pupils to suggest ways in which we use artificial satellites, and to use secondary sources to find out more about these, eg meteorological, communications, scientific research, telescopes, observatories.
• Provide starting points for activities, eg
• Produce a time line showing the main events in satellite technology. Ask pupils to consider What is the Global Positioning System (GPS) and how does it work? What impact have satellites had on everyday life?
• Make models or diagrams of satellites and find out what their different parts are for. Ask pupils to consider How are weather satellites used? What is a space station used for? How do scientists interpret information from satellites to make predictions?
• Ask pupils to find out whether the satellites studied are geostationary or in polar orbit, and explain the significance of this. Discuss with pupils how information from satellites is sent back to Earth, emphasising the large distances.

• Review pupils' understanding of work in this unit by asking them to work in groups, using their books, to make a list of five or six key points about gravity and space. Ask pupils to present their points and agree a summary of them as a class.