Home Experiment: A law of physics with an unexpected twist

This is an extract from our newest title

‘ Everyday Physics -Unusual Insights into Familiar Things’ . This is a physics book like you’ve never seen before. It’s accessible and fun, for all ages – perfect for anyone with a healthy dose of curiosity.

This particular experiment helps us to understand how we move and walk in the way that we do.

With an office chair or a bar stool that swivels smoothly, you can give two nice demonstrations of the law of conservation of angular momentum. (The chair must swivel very smoothly – lubricate it to make sure.)
Demonstration 1.
This illustrates the pirouette principle: the more mass we move inwards towards the axis of rotation, the more the angular speed increases. (You can see this very clearly with a rotating figure skater. When their arms and one leg are outstretched, they rotate slowly, but as they draw in their limbs, they rotate faster and faster.)

Raise the seat of your swivel chair to its highest position (to keep your feet off the floor).

Spread your hands horizontally and get someone to spin you and the chair (Fig. 1a).

Now bring the two bottles towards your chest or – better still – above your head, because that is as close as possible to the axis of rotation (Fig. 1b): your rotational speed increases.

If it goes too fast you can slow down by extending your arms again.

Fig 1a Spread your hands containing bottles of water and get someone to spin your chair.
Fig 1b Raise the bottles above your head and your rotational speed increases.

This is the law of conservation of angular momentum in action.

During these operations, there is no external force, so the amount of rotation –the angular momentum – remains constant. However, it depends not only on the rotational speed, but also on the distribution of mass around the axis of rotation. As a consequence, if the mass gets closer to the axis, the rotational speed must go up to compensate. We see the same phenomenon in tornados: as the moving air gets closer to the “eye” of the tornado, its rotational speed increases.

WHY DO WE
WALK THE WAY
WE DO?

The way you walk seems normal because everybody does it. You swing your arms and legs in opposite directions; when your left leg goes forward, your left arm goes backward. Same thing on the right.

Why do you do that? Because you unconsciously take account of a law of physics – that the angular momentum (the amount of rotation) of a body remains constant in the absence of external forces. Your arms and legs are not attached to the middle (vertical axis) of your torso, but to the side. When your right leg moves forward it must push itself off from the right side of your body, so the right side of the body is pressed backward (just as there is recoil if you shoot a gun from your right shoulder). The consequence is that you tend to turn around your vertical axis. You prevent that by moving your right arm backward at the same time, in compensation.

You could move your left arm and left leg forward or backward at the same time, but then the movement of the arm wouldn’t compensate for the movement of the leg; instead it would make the rotation worse. This would happen at every step, and your feet would have to absorb this rotation in their contact with the ground. So, walking this way not only looks funny, it’s inefficient too.
The same consideration applies when you walk up the stairs with a cup of tea, to surprise your partner in the morning. As you walk up the stairs with the tray in your hands, you inadvertently move the tray to and fro, because you miss the natural swing of the arms. Thus, a piece of applied science prevents you from spilling tea all over the place.
Ten-pin bowlers are especially troubled by this problem when throwing their heavy bowling ball. However, experienced bowlers have found a solution: just as they release the ball, they automatically make an elegant sweep with their free leg to compensate for the torsion. The law of physics provides an unintended bit of sporty elegance!

Watch out for other posts showing the laws of physics in action in our daily lives.

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