A two-legged robot like Zowi has to be very careful where he puts his feet, because if he takes a wrong step he can lose his balance and fall to the ground!
This happens because, as you already know, there is a force that attracts things to each other called gravity. It is the force that makes us stick to the ground and also the one that makes planets and stars attract each other, like the Sun and the Earth.
The Earth also attracts you, just as it attracts Zowi. But you don’t always fall to the ground, do you? This is because there is also something we call the center of gravity, a reference point that helps us know if an object can fall or remains in balance. We are going to learn how to know where the center of gravity of an object is.
Did you know…?
Actually, it’s not just the Earth that attracts you. It’s hard to imagine, but all the things around you right now are also drawing you to them and you are drawing them to yourself. What happens is that they attract you very very very little, so little that you don’t notice it.
Experiment 1 – The “Rule” of Gravity
Let’s start by understanding where the center of gravity is on a ruler.
Its center of gravity is right at its midpoint. You know why? Because it is the exact place where neither of the two halves weighs more than the other.
Let’s check it out:
Take a rule that you have at home.
Rest the ruler on the edge of the table so that most of it is inside the table. Then slowly move him out and see when he loses his balance and falls.
How many centimeters did the ruler mark when you fell? You will see that when the fulcrum exceeds the midpoint of the ruler, the force of gravity pulls it towards the ground!
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When the center of gravity becomes unsupported, gravity pulls it downward, causing the ruler to drop.
Did you know…?
Gravity is said to have been discovered by a scientist named Isaac Newton about 350 years ago. Newton was leaning against an apple tree when he saw an apple fall in front of him. Suddenly it occurred to him that all things attract all things and he created the Theory of Gravity.
However, humanity still knows very little about this mysterious force, and other scientists such as Albert Einstein have made great discoveries. The last great discovery occurred very very recently, in February 2016.
Experiment 2 – Zowi’s gravity
Now, we are going to do a similar experiment with Zowi. Zowi’s head weighs more than his legs, so his center of gravity is not exactly at his center, but is more or less between his eyes.
Again, this is the point that tells us that both sides of the object weigh the same, so if it is tilted it will not fall towards either side.
Let’s check it out!
Take Zowi and turn him off.
When Zowi’s center of gravity is between his two legs, he is stable.
Slowly tilt it to one side.
Even when his center of gravity is aligned on one foot, Zowi is still balanced.
But, when the center of his head is no longer above the support zone (red zone), Zowi falls to the side!
The same thing happens to you. Try to imitate Zowi! Your center of gravity is roughly at the level of your belly button. You will see that as soon as this point is out of line with your fulcrum (that is, your foot), you fall to the side.
The center of gravity is a very important factor to take into account in the design of a robot, since it affects its movement. If, while moving, the center of gravity leaves the fulcrum, the robot will fall.
Find the center of gravity of other objects you have around the house and see how what you have learned is true. Can you balance and hold a long stick over your open hand? When does the stick fall vertically when balancing?