Experiment: Relativity And Roundabouts

This is a very simple, but fun set of experiments that can be used to introduce your child to the basic ideas behind Einstein’s theories of relativity and the physics of movement. Even if they don’t fully understand the theory behind it (and let’s face it, who amongst us does!), they will learn something new about the world around them and how it works.

Relativity and Roundabouts

Experiments on roundabouts are a fun way to introduce your child to the concept of relativity and the science of movement.

What Will You Need? To do this set of experiments, all you will need is a playground with a roundabout. Ideally, the playground shouldn’t be too busy so that you and your child can mess around on the roundabout without other children getting in the way.

How Long Will It Take? The actual experiments will only take a few seconds each, but you may well have to repeat them over and over again as your child will probably enjoy doing them a lot (because they get to spin round on a roundabout very fast!).

What Age Of Child Can Do This Experiment? This set of experiments is suitable for children aged four and older, although you will need to tailor your explanations of the theory behind it to the age and intellectual development of your child

What Are The Experiments? There are a range of experiments which you can try with your child on a playground roundabout, and each will illustrate a slightly different scientific principle of movement. These are:

  1. Relativity: Einstein’s theory of relativity states that you can only perceive things as moving relative to something that is outside of that object’s frame of reference. Thus, if everything around the object is moving at the same rate, then we cannot perceive the movement. This is the reason why we cannot feel the Earth rotating beneath our feet, or orbiting the sun, or our solar system moving through the galaxy, or our galaxy moving through the ever-expanding universe. To investigate this, sit on a roundabout with your child and rotate it as fast as your can. Get them to focus their attention on you, and wave your arms around. Then ask them if your arms are moving. They will undoubtedly say yes. Now keep your arms still and ask them whether your arms are still moving. They will most likely say no, because they are not moving relative to the roundabout. If they do say no, ask them if the roundabout is still moving and then ask again if your arms are moving. This demonstrates that the perception of movement will depend on the frame of reference you are comparing it to. If you wish to take this experiment further, you can stop the roundabout and, while keeping your arms still, ask them if your arms are moving. Again, they will most likely say no. If they do, you can explain to them about the Earth rotating, so that you arms are still moving, just not relative to the roundabout, or the Earth, but in relation to the wider solar system. At this stage, be prepared for quite a bit of confusion and questioning!
  2. Who is moving: Another element of relativity is that when two objects are moving apart, for an observer placed on each object, it will always appear to be the other one that is moving, and both are quite correct, for their own frame of reference. This can be demonstrated by standing next to the roundabout while your child is spinning round on it. Ask them to explain how they know who is moving, them on the roundabout, or you standing beside it.
  3. Mass and acceleration: A third aspect of relativity is that as things change their relative speed, so does their mass (how much they are perceived to weigh). This is the reason it feels like you are pushed back in your seat when you accelerate a car fast away from a set of traffic lights, but why when you are cruising at sixty miles an hour you don’t feel any such pressure. You can get the same effect on a roundabout by facing the direction of the spin and holding your arms up above your head (if you do this on one side of the roundabout, your child can do it on the other and follow your movements to feel the same effects). Once the roundabout is going nice and fast, lower one arm towards the centre of the roundabout, it will now be moving slower than your body and will feel lighter. Raise the arm back up and lower the other one away from the centre of the roundabout. It will now be moving faster than the rest of your body, and it will feel much heavier, and will be much harder to raise up again. Next, lower both arms at once so that they are pointing in opposite directions and try to clap your hands in front of you. This will demonstrate the difference between the two arms created by the differences in their relative speeds. It will also show how it takes more force to bring your arms forward (to go faster than the roundabout itself) than backwards (which is slower than the roundabout). You can extend this basic experiment by moving arms in and out, back and forth and in and out in all sorts of directions and seeing how it makes you feel.
  4. Conservation of momentum:  This one is a bit trickier to do, and requires the type of small roundabout which only one person can fit on. Alternatively, you can use the type of office chair that spins round. Start by getting your child spinning round. Once they are up to speed, get them to reach out their legs (but make sure that they won’t hit anything first!). They will feel themselves slow down. Now get them to draw their legs in and they will feel themselves speeding up again. This can be repeated over and over to change the speed of the spin. The reason this works is that the momentum created by spinning is maintained at all times. When you reach your legs out, they have to move further to complete each circle, this means they have greater acceleration, which in turn increases their mass. To maintain the momentum of this increased mass, overall the rotation slows. When you pull your legs in, they have to move a shorter distance to complete each circle. As a result, their acceleration is slowed, decreasing their mass, and the overall rotation must speed up to maintain the original momentum. This is a bit complicated, but at least it’s fun to do! You can find out more about what’s going on in this experiment here.

What Core Life Skills Will It Help Develop? This experiment will help your child develop an understanding of how the world around them works, and specifically the concepts of relativity and the physics of movement. It will also help them to think about the science behind simple everyday actions.

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About The Author: This post was written by Colin Drysdale, the creator of How To Raise A Happy Genius.

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