Orbits you can feel: string, ball and funnel

Orbits are hard to picture because nobody can stand next to one. These three demos let learners feel the ideas behind Orbit Keeper with ordinary things. Each takes about 10 minutes. Try them before or after a few game missions.

The big ideas

• An orbit is a fall that keeps missing. Gravity pulls, and sideways speed carries the object past.
• Higher orbits are slower.
• Too slow means falling in. Too fast means flying away.
• Astronauts float because they are falling, not because there is no gravity.

Demo 1: The ball on a string

You need: a light ball (a tennis ball in an old sock works well), 1 metre of string, and a clear outdoor space with nobody nearby.

Do it:

1. Tie the string to the ball. Swing it in a slow, flat circle above head height. Keep everyone well back.
2. The string is the "gravity". Feel the string tug on the hand.
3. Swing faster. The tug gets stronger. Ask: what happens to the string if the ball goes more slowly?
4. Let go (aim away from people). The ball does not fly outwards. It flies in a straight line along the direction it was moving.

Ask:

• What does the string do? (It pulls the ball inward, like gravity.)
• Why doesn't the ball fly into your hand? (It is moving sideways.)
• What would happen to a satellite if gravity suddenly switched off? (It would fly off in a straight line, just like the ball.)

Safety: use a soft, light ball, check for people behind and around you, and let one person take charge of releasing the ball.

Demo 2: The funnel and the marble

You need: a large funnel or a round salad bowl with a smooth inside, and a marble.

Do it:

1. Hold the funnel steady. Roll the marble gently around the inside edge so it circles.
2. Watch the marble. It circles, then slowly spirals down towards the middle.
3. Try a faster roll for a bigger circle. Compare the number of laps in 10 seconds.

Ask:

• Why does it spiral inward in the end? (Friction slowly takes away its speed. Too slow means falling in.)
• Which circle takes fewer laps, the wide one or the narrow one? (The narrow, lower one is faster. Close-in satellites zoom round.)

Link to the game: In real life, a satellite very close to Earth feels a tiny bit of air. That slowly drags it lower, and the Space Station needs little pushes from its rockets to stay up. This is why Mission 1 warns you about dipping into the air.

Demo 3: Throw it faster

You need: a ball, a big open space, a tape measure or lots of steps.

Do it:

1. Throw a ball sideways, gently. Measure how far it goes before it lands.
2. Throw it as hard as you can. It goes farther.
3. Now imagine throwing it so hard that it flies a whole 8 kilometres. In that distance, Earth's surface curves down by about 5 metres. So a ball moving 8 km per second would fall 5 metres in that one second, and the ground below would have curved away by the same 5 metres. It would never land.

Ask:

• What do you need to change to make the ball go farther? (Throw harder, which means more speed.)
• What would stop a real ball from going all the way round? (Air and the ground in the way.)
• Why can a satellite go all the way round? (It is high enough that there is almost no air.)

Questions that match each mission

Words to introduce

Orbit, gravity, satellite, ellipse (oval), escape speed, free fall. If you are teaching beginners, keep the first two or three.

Common misconceptions

• "There is no gravity in space." There is plenty. At the Space Station it is about 90% as strong as on the ground.
• "Satellites are held up by something." Nothing holds them up. They are falling around the planet.
• "Faster satellites are higher." It is the other way around. Higher satellites go slower.