## Question 10: A central force is one that is always directed towards the same point. Can a central force give rise to a torque about the point?

Solution

No, a central force cannot give rise to torque.

This is because it is antiparallel to the moment arm at that point.

We know torque is the vector product of force and moment arm,

Therefore, it depends upon the sine value of the angle between r and F. Now, when they are antiparallel, the angle between them is 180° and its sine is 0. This makes the torque zero due to the central force.

### What is central force?

Central force is a concept of classical mechanics and you will read more about this in higher classes. For the time being, it is the force that is radially pointing and the magnitude is dependent on the distance from the source. As an example, you can take the earth’s gravitational force. You know it is acting radially and depends upon the distance from the center of the earth.
Do you know what a radial force is? Well! A person in America is probably on the other side of the earth (in relation to Pakistan). Still the earth attracts him/her towards itself (to the same point) as the earth do it here! Now you should develop your idea about radial forces. Other example is of electric force of a charge.