balance

Centre of gravity

Throw a ball in the air and gravity pulls it straight back down. Not everything moves like this when gravity acts on it. Most objects are not nice, neat shapes like balls. That means gravity acts on them in more complex ways. Even so, all objects behave as though their mass (the stuff they're made from) is concentrated at a point called their center of gravity. A simple object like a ball has its centre of gravity in a very obvious place: right at its centre. But in a more complex object, like your body, the centre of gravity is slightly higher than your waist because there's more weight in the top half of your body than in the bottom half.

How does it help to know about centre of gravity?

A tightrope walker has an instinctive command of physics. You might think the fan she's carrying is just a funny little touch, but it's more important than it looks: it gives the walker an easy way to redistribute her weight and quickly correct any wobbles that could send her to the ground

A helicopter carrying a side load has its centre of gravity shifted to the left, as we look at the picture. The pilot has to adjust the pitch of the rotors so they create more down-force on the left to compensate.

If you want to fly an aircraft safely, having a balanced load is important. This giant C-5 Aircraft is having its centre of gravity calculated in a special weight and balance hangar.

Thinking about center of gravity is also key to playing many sports effectively. Anything that involves balance—pretty much every sport from figure skating to surfing—involves thinking about where your weight is and how to move it quickly without using too much energy or losing control. You've noticed how tennis players plant their feet wide apart? And how high jumpers do weird things curling their bodies up and round the pole? All that kind of thing is based on understanding centre of gravity—and putting it to practical use!

Why does gravity make your body tip over?

Imagine your body is not a single, solid mass but a huge sack of potatoes standing upright. Gravity pulls on the whole sack, but it also acts on each potato separately, pulling each one downward. When you lean over to one side, the "potatoes" at the top of your body work like a lever, making your top half turn and topple about your ankles. The more you lean, the bigger the lever effect at the top of your body—and the more likely you are to topple.

There's another way of thinking about your weight. Yes, your body is a bit like a sack of potatoes. But it's also a bit like one giant potato, weighing as much as you do and concentrated in an infinitely tiny point, somewhere in your middle—roughly where your stomach is. This is your own, personal centre of gravity. As long as your centre of gravity is more or less above your feet, your body will always be balanced and you won't tip over. But start leaning to the side, and everything changes. Your head is one of the heaviest parts of your body—like a giant potato perched right on top. If you lean to your left, your centre of gravity is no longer directly above the midpoint of your feet. The more you lean, the more torque (turning force) this creates and the more likely you are to topple over. Gravity makes your whole body rotate about your ankles like a finger pushing on a door handle.

What's the best way to balance?

The lower your centre of gravity, the easier it is to keep your balance. If you're sitting on a chair, you can lean over more than if you're standing up. With your centre of gravity low, you can lean further to one side or the other without creating enough turning force to tip you over. That's why racing cars (and military vehicles) are designed with very low centres of gravity: the lower they are to the ground, the less risk there is that they'll tip over, no matter how fast they go.

Tightrope walkers use a slightly different trick to master their centre of gravity. If you've ever watched a tightrope walker, you'll have noticed they never simply walk across the rope. Some stretch their arms out or carry a long stick or an umbrella. Others crouch down or bend their knees. Still others ride bicycles with weights dangling some way beneath them. These balancing aids help to give tightrope walkers more control over their centre of gravity. If they can keep their centre of gravity directly above the rope at all times, they will never fall off. If they start moving to one side, a turning force will start to topple them in that direction. So they have to quickly move part of their body to the other side to make a turning force in the opposite direction and restore their balance.

If you're a sceptic, you might think science is full of useless bits of information you never really need to know, but centre of gravity isn't one of them. Last winter, the lane where I live froze over completely and turned to a sheet of ice. What's the best way to walk down a frozen street? Assuming you don't have mountaineering boots, the safest way to do it is to get down on all fours and crawl along, like a polar bear, on your hands and knees. You might get wet or dirty but you won't tumble over and break your neck. If you make your centre of gravity very low, it's impossible to fall.

Centre of Gravity Activities

Balance a ruler with a hammer.

Take a rubber band or string and make a loose loop around the hammer and ruler, as shown in the picture. Make sure the end of the hammer is touching the ruler, and then position the ruler at the edge of a table, as shown. (You might have to re-position the string/rubber band a few times to get it just right). Why does this trick work? Analyse where the centre of mass might be. Where is the balance point? What is the heaviest part of a hammer?

Balancing forks

You need 2 forks, a £2 or wooden stick, a cup, and some patience.

You could probably guess where the centre of gravity is for a lot of symmetric objects. For example, a ruler’s is in the middle. But if you add some extra weight to the end of a ruler, its centre of gravity will shift!

If you try to balance 2 forks on your fingers, you probably won’t win. This is where things get weird– the centre of mass does not necessarily need to be within the object! For 2 forks, it ends up being just right outside of it.

What makes this “trick” work? If you stick something, like a £2 coin, between the 2 forks, the centre of mass of this collection of objects is now on the £2.

You will intuitively find the exact place on the coin where the centre of mass lies and when you do balance on something like the edge of a cup !

(tip: the more rigid the edge of the glass is, the better. We used the bottom of the cup here because that was less rounded, and hence stuck better, than the top of the cup). There you have it!

Uncanny Cancan

Pour about 100ml of water into an empty soft drink can
Tip the can on an angle and try balancing it
The can will balance when the two parts of the rim shown here are touching the table
Let go of the can and it stays balanced – it looks precarious but you will find it’s actually remarkably stable!

This picture might help you get your head around what’s going on inside the can – the red dot represents the centre of gravity of the can when it’s full or empty. The blue dot represents the combined centre of gravity of the can plus the water in it.

Chair balancing on One leg

Centre of Gravity Challenge 1: The Thumb Press

Place a chair against the wall so that it cannot slide backward.
Sit in the chair with your feet flat on the floor in front of you. (Your feet may not be angled or slanted to the side.)
Have a partner gently place a thumb in the middle of your forehead.
Now try to stand up without forcing your partner’s hand back.

WHAT HAPPENED? The reason it’s so hard to stand up is because your centre of mass is located over the seat of the chair rather than over your feet, which are in front of you.

Centre of Gravity Challenge 2: The Penny Grab

Stand with your back and feet against the wall. (Your legs must be straight and feet together.)
Have a partner place a penny between your feet, in front of your toes.
While keeping your legs straight, bend over and pick up the penny without falling over.

WHAT HAPPENED? The reason this challenge is so difficult is because of our anatomy—specifically, our bottoms. As you bend over to pick up the quarter, your rear end naturally extends backward to help keep your body balanced. Since it’s pressed against the wall, your bottom has no where to extend. This causes the centre of mass to shift forward, resulting in falling forward—at least for most people.

Centre of Gravity Challenge 3: The Leg Raise

Stand with your right shoulder and right leg pressed against the wall.
With both your shoulder and leg touching the wall, “simply” raise your left leg. Your goal is to stand only on your right leg—the one that is touching the wall.

WHAT HAPPENED? This challenge is nearly IMPOSSIBLE. For the same reason as before: because the wall is in the way, your body can’t counter-balance the mass distribution. Your centre of mass is over BOTH legs, not just one.