The centre of mass is the single point where the entire mass of an object is considered to be concentrated. Imagine an object as a collection of many tiny particles. If you could balance the object at one specific spot so that it does not tilt, that point is the centre of mass.

Where Mass Meets Balance

For a symmetrical object like a uniform wooden ruler, the centre of mass is exactly in the geometric middle. If you push the object at this point, it will move in a straight line without spinning. If you push it anywhere else, the object will start to rotate. This point depends only on the distribution of matter inside the object.

The Role of Gravity

The centre of gravity is the point where the total weight of an object acts. Weight is the force exerted on an object by gravity. In most places on Earth, the centre of mass and the centre of gravity are in the same location. This is because gravity acts almost equally on every part of a small object.

When are they different?

• Uniform Gravity: For small objects on Earth, the centre of mass and centre of gravity are identical.
• Large Objects: For massive structures like a skyscraper, gravity is stronger at the base than at the top. In these rare cases, the centre of gravity might shift slightly compared to the centre of mass.
• Space: In deep space, where there is no gravity, an object has a centre of mass, but it has no centre of gravity because there is no weight force pulling on it.

Real-World Examples

Consider a high jumper clearing a bar. A skilled athlete can arch their back so that their centre of mass actually passes below the bar while their body goes over it. This allows them to jump higher with less energy. Balancing a spinning basketball on your fingertip works because your finger is placed exactly under the ball's centre of mass. When these points are aligned, the object remains stable and does not topple over.