Have you ever thought why the Tower of Pisa doesn’t fall? Despite being crooked, the tower does not fall, since the vertical of its centre of gravity (vertical line drawn from the centre of gravity) is inside the base. But do you know what the center of gravity of a body is? The center of gravity is the point of application of the result of all the forces of gravity acting on a body, and the stability of a body depends on the position of the center of gravity in relation to the base. That is, if from the body’s centre of gravity (in black in the image below) we draw a vertical line (in red in the image below) that lies within the base of the body, the body remains standing. If, on the contrary, it is outside the base, the body falls.

Thus, a body will be more stable the lower its center of gravity and the wider its base. Have you understood the concept of center of gravity? To see if it’s true, confirm it with these challenges. Let’s play with the centre of gravity!

To complete the challenges on this sheet, you need the following materials:

  • A chair.
  • An egg.
  • A device connected to the Internet: telephone, computer,

The challenge of the chair:

  1. Sit in the chair: sit in the chair like the girl in the picture below (legs and back must be upright):
  1. Getting up from the chair: While sitting, keeping your legs and back in an upright position without using your hands, are you able to get up from the chair?

What happened?
If you do not cheat, in these conditions it is not possible to get up from the chair without falling. Since the vertical that passes through the centre of gravity of your body is in the centre of the base of the chair, it is not possible to get up without losing the verticality of your back and legs. Therefore, if the girl in the image is sitting as she is, the inclination of the body forward is fundamental to get up without falling, since by leaning forward the centre of gravity of the body changes.

The chair challenge II: Chair challenge

To perform this challenge you must follow these steps:

  • Stand in front of a wall and back away three steps (keep chair handy).
  • Tilt your body forward 90 degrees until your head touches the wall.
  • Without changing your position, take the chair with both hands and approach your chest.
  • Try to lift the chair without letting go.
    In the following link you will find an example of this challenge:
    This challenge is very interesting if all of you at home do it. This way you can see if you are able to complete the challenge according to the characteristics of each body.
  1. Look for an explanation of the difference between the difficulty of the different people to complete the challenge: What happens? Do the tallest and the shortest challenge the same? Does it have to do with the size of the trunk? And does the size of the feet have to do with the ease of completing the challenge? Think and try to find an explanation. Does it have to do with the centre of gravity?

What has happened?
To maintain balance, a person’s center of gravity must be above the area occupied by the feet. Otherwise, it will tend to fall. When we tilt the body 90 degrees to touch the wall with our head, the center of gravity moves forward, so if it is outside the vertical projection of the feet, it will be more difficult to succeed in the challenge.
The center of gravity of people is in the pelvis, depending on the size of the trunk, sometimes a little higher than others. In addition, those with larger feet move further away from the wall than those with smaller feet when challenged. Therefore, when getting into the required position in the challenge, the center of gravity of the larger logs moves forward more than that of the smaller logs, and when grabbing the chair the center of gravity is not on the feet, failing the challenge. In the case of people with the lowest center of gravity, however, the center of gravity is not so far forward and stays on the feet, even after the chair is lifted, so they can successfully complete the challenge. Success in achieving the challenge depends on the anthropometric characteristics of each individual.

The egg challenge:

  1. Take a raw egg and try to put it upright: As shown in the picture below, try to put the egg upright.

Shake strong egg: Take the egg and after shaking it with a strong hand, try to put it back in the vertical position.

What happened?
The centre of gravity of the egg is approximately in the centre (in the yolk) and its base is very small so it is very difficult to stand. When you shake the egg, the yolk drops to the base and standing is easier, since its centre of gravity is very close to the base.

OpenEdition suggests that you cite this post as follows:
danelsolabarrieta (July 7, 2020). THE KEY TO BALANCE. SPOON Open Notebook SPOON . Retrieved July 13, 2024 from

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