# (ELI5) What is the center of gravity?

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(ELI5) What is the center of gravity?

In: 9 First off that’s isn’t biology.

But basically the center of gravity is like a magic spot on an object where all the weight is piled up. It’s a special spot that tells us where the object is heaviest. Sometimes the magic spot is in the middle of the object, but sometimes it’s not. We can find out where the magic spot is by hanging the object from two different places and seeing where it balances. Scientists have special books that tell them where the magic spot is for different shapes of objects. Does that make sense? Cut an arbitrary shape out of thick card. Poke a hole in it near the edge and hang it from a piece of string through the hole. Draw a line vertically downwards from the hole to the other side of the shape. Now poke another hole elsewhere around the edge and repeat the process. After a few goes you should notice that all the lines you draw meet at one point. This is the centre of gravity of the shape.

The same idea applies to any 3D object; there will be some point which will always be under the suspension point. For an animal this is slightly complicated by the fact that they can change shape.

If you stick your arms out forward then you need to lean back slightly to keep your centre of gravity over your feet; otherwise you would fall over forwards.

Where the centre of gravity is will affect how stable something is, or where you should push the object to avoid tipping it over. The center of gravity is essentially the same thing as the center of mass.

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If you have some spherical object with the same density in it, the center of mass will be in the middle. But if say the left side is heavier than the right side, that center of mass will be more on the left side than the right.

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When you are dealing with gravity all mass will pull slightly towards them, but doing all these calculations is impossible. So what you do instead of calculate the center of mass, and you can mathematically prove that the calculations for gravity end up the same if ALL the mass was just located at the center of mass, instead of being spread all over the place. The center of gravity is just the average location of all of a thing’s weight.

Gravity acts on mass. Every piece of an object has mass and is pulled on by gravity (producing weight).

Figuring out how gravity affects something with a complicated shape can be tough. One trick is to imagine the object is actually just a single point with the same total mass.

Of course, you want this point to be located where the average of all of the object’s mass is – and that’s the center of gravity.

So for example, if you want to figure out whether someone is going to fall over or not, you can just check whether their center of gravity is over a part of their feet. If it’s not, they’re going to fall.

It’s effectively a summary of where gravity is applying force to an object. Centre of gravity and centre of mass are essentially equivalent in most practical circumstances.

Centre of mass is an an imaginary point where an objects’s inertia acts. The position is determined by the location and density of all of the object’s connected masses. Applying a force to the object’s centre of mass moves it without inducing rotation.

Centre of gravity is an imaginary point through which the collective gravitational force of an object’s mass acts. Here on earth, the top of an object experiences less gravitational force than the bottom, meaning it’s centre of gravity is below its centre of mass.

The distinction is meaningless for small objects here on Earth. However, large objects space will rotate if their centre of mass and centre of gravity do not align along the axis of gravitational force acting through the centre of gravity.