Question 1

What is the centre of mass?

Accepted Answer

The centre of mass is the mass-weighted average position of all particles in a system. For external translational motion, the whole system can be treated as if its total mass were concentrated there. It does not remove the body's size, shape, or rotational behaviour.

Question 2

How does the centre of mass divide a two-particle system?

Accepted Answer

For two positive masses, the centre of mass lies between the particles on their joining line. If their distances from the centre of mass are d1 and d2, then m1 d1 = m2 d2, so the line is divided inversely as the masses.

Question 3

Why does the plumb line pass through the centre of mass of a suspended body?

Accepted Answer

In equilibrium, the tension acts at the suspension point while gravity acts effectively through the centre of mass. The body rotates until the centre of mass lies vertically below the suspension point, so the plumb line through the suspension point also passes through the centre of mass.

Question 4

Where is the centre of mass of a uniform triangular lamina?

Accepted Answer

It is at the intersection of the three medians. This point lies two-thirds of the median distance from each vertex and one-third of the distance from the corresponding opposite side.

Question 5

Can the centre of mass lie outside the material of an object?

Accepted Answer

Yes. A uniform open ring-arc can have its centre of mass outside the arc material. Balance on a knife edge is still possible when the knife contacts actual material whose vertical line passes through that centre of mass.

Centre of Mass Lab — Find the Balance Point in 3D

About this lab

Practice