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Gravity well

By David Shaw, 11 October 2018

Have you ever wanted to experiment with the orbits of moons and planets? Time to make a gravity simulator!

You will need

• Hula hoop – bigger is better
• Very stretchy fabric, enough to cover the hula hoop – we used a polyester blend with a high percentage of elastane/lycra/spandex
• Cloth tape or gaffer tape
• Scissors to cut tape
• Heavy ball, e.g. hockey ball
• Lighter ball, e.g. squash ball or high-bounce ball

What to do

1. Find an open area and lay out your fabric.
2. Place the hula hoop flat on the fabric, so that the fabric covers the area inside the hoop.
3. Cut a piece of tape about 12 centimetres long and use it to tape the fabric directly onto the hula hoop. Leave a gap of about 5–10 centimetres and then place another piece of tape so it holds the hoop to the fabric. Keep taping until you make it all the way around the hula hoop.
4. Turn the whole thing upside down.
5. Arrange some chairs or tables in a circle so the hula hoop can rest on them. You want to leave space under the fabric.
6. Put the heavy ball in the middle of the fabric. Then, try to roll the light ball in a circle around the heavy ball. How did you go?
7. Put the light ball in the middle of the fabric. Try to get the heavy ball to roll in circle around the light ball. Did it work better with the light ball or the heavy ball in the centre?

What’s happening?

In our everyday, earthbound lives, gravity’s pretty simple. Stuff falls down. But if you look at stars, planets and moons, things aren’t so simple.

The force of gravity acts between all objects, all the time. The strength of the pull is based on two things – how close together the objects are, and how massive they are. It’s working like this on Earth too, it’s just that our planet is much more massive than everything else nearby, and much closer than anything bigger.

This activity illustrates one way to think of gravity in space. Around each ball in our simulator, there is a depression in the fabric. In space, every object has a ‘gravity well’, an illustration of its gravitational field.

The slope of the well tells you how hard the pull of gravity is at that point – so it gets stronger closer to the ball. The depth of the well tells you how much energy it takes to escape, once again, much more when you are closer to the object.

In this activity, the two wells interact with each other. When a lighter object is introduced to a heavier one, it might start to orbit. This happens in real life too – both of Mars’ moons are thought to be captured asteroids.

When a heavier object encounters a lighter one, it’s not as nice a story. The lighter object can be flung wildly, down towards the Sun or out into space. Some planets can get ejected from their solar systems entirely and become rogue planets. And these aren’t just a theory – astronomers have spotted them out in space.

There can also be positive uses for these violent flings. Spacecraft can use gravity wells to perform a gravitational slingshot manoeuvre. In essence, they fly close to a planet and use its gravity to speed up, slow down or change paths. The Parker solar probe will use seven slingshots on its trip towards the Sun, and Voyager 1’s encounters at Jupiter and Saturn mean it now has enough speed to escape the solar system entirely.

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