Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• introduce the wide variety of objects that function as satellites
• draw out ideas of what a natural motion might be
• explore expectations about satellites falling back to Earth
• draw out any difficulties with the motion of satellites
• introduce a wide range of uses for artificial satellites
• include natural satellites alongside artificial satellites

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• account for circular motion
• explain the need for a centripetal force
• distinguish between orbiting and spinning on an axis
• draw diagrams to distinguish between geostationary and circumpolar orbits for satellites

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Staying in orbit

Wrong Track: Something must keep satellites up in the air, they can't just float in space, they're big heavy things. I mean aeroplanes have jet engines and wings to keep them up.

Right Lines: The satellite is taken up to its orbital height on a carrier rocket and directed out along its orbit, at a specific speed, with a rocket thrust. This speed is such that the gravitational force, at that height above the Earth, provides the centripetal force needed to keep the satellite in its orbital path. If the satellite is sent out into its orbit too slowly, it will fall to Earth. If it is sent out too quickly, the gravitational force will not be big enough to hold the satellite in orbit and it will fly off into space. Nothing keeps the satellite up. It is just a matter of setting the speed of the satellite such that the gravitational pull of the Earth (at the given height) tugs it around in its orbit.

Use a thought experiment

Thinking about the teaching

You might try using Newton's thought experiment, of throwing a ball into orbit from a huge mountain to transfer this idea that the satellite is continuously falling towards the Earth and nothing is needed to keep it up.

Circular motion

Wrong Track: Something must keep satellites going. Don't they have rockets which push them around their orbit?

Right Lines: Once the satellite has been accelerated into its orbit by a rocket thrust, it continues along that orbit, being tugged into its path by the gravitational pull of the Earth. In empty space the satellite would neither speed up nor slow down, but would keep travelling along its orbit forever. In low satellite orbits where there is a thin atmosphere, the air creates a drag and slows down the satellite. In such cases there is the need for occasional rocket thrusts to maintain the motion of the orbiting satellite.

There's a good chance you could improve your teaching if you were to:

Try these

• giving time to exploring the idea of a natural motion
• giving physical experiences of motion in a circle
• explicitly modelling the two dimensional motion, if you go that far
• building up to the idea of an orbit, from thrown objects, gradually increasing the launch speed

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• conflating the launching of a satellite with is orbiting
• restricting the discussion of satellites to artificial satellites
• centrifugal force

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.