Planet Hop

Little levels (ages 4 to 6)

1. Jump on the Moon Jump on Earth, then on the Moon, and compare the ruler.
2. Heavy or light? A bear weighs one on Earth; predict Jupiter and the Moon.
3. Which planet? Jump on a mystery world and guess from the height.
4. Feather and hammer Drop both on Earth, then predict what happens on the Moon.
5. Big pulls more Tap the world with the strongest pull and the weakest.

Big levels (ages 7 to 9)

1. Weigh yourself everywhere Set your weight and tap the Moon, eight planets and the Sun.
2. Jump race Order the Moon, Mars, Earth and Jupiter by jump height, then race.
3. Which way is down? Kids on every side of a round world: where does a dropped ball go?
4. Newton's cannonball Find the speed that turns a cannonball into a satellite.
5. Rocket size Order the worlds by the rocket you need to leave them.

The big ideas

• Gravity is a pull between things with mass. Everything pulls on everything, but only huge things like planets pull hard enough to notice. The more mass a world has, the stronger its pull at the surface.

• Weight is the pull; mass is the stuff. Your mass is the same on the Moon, but you weigh about a sixth as much there because the Moon's pull is weaker. On Jupiter you would weigh two and a half times more.

• Everything falls at the same rate. Without air, a feather and a hammer land together. On Earth, air resistance slows light, wide things down.

• Down means toward the center. Gravity pulls toward the middle of a world, so people on the far side of the Earth are not upside down and nobody falls off.

• An orbit is falling and missing. A satellite falls toward Earth all the time, but it moves sideways so fast that the ground curves away beneath it. Rockets have to reach about 11 kilometers per second to leave Earth altogether.

Try it at home

1. Measure your jump Jump next to a wall and mark the highest point. Multiply by six for the Moon and divide by two and a half for Jupiter.
2. Paper and book Drop a flat sheet of paper and a book side by side, then put the paper on top of the book and drop them again. Without air pushing on it, the paper falls just as fast.
3. Bucket swing Swing a bucket with a little water in it over your head in a fast circle, outdoors. The water stays in, the same way a satellite stays up.
4. Scale trip Weigh yourself, then work out your weight on the Moon (divide by six) and on Jupiter (times two and a half).

Ask after playing

• Why did you jump higher on the Moon?
• What happened to the feather when there was no air?
• How can a cannonball keep going round and round?