Earth and Space | Forces and Motion

What keeps satellites up?

Teaching Guidance for 5-11 11-14 Supporting Physics Teaching

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.

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