4.3 Turning Effect of a Force

4.3 Turning Effect of a Force

We have learnt that a net force affects the linear motion of an object by causing it to accelerate. Since rigid objects can also rotate, we need to extend our concept to the turning effect of a force.

When we open or close a door, we apply force. This force rotates the door about its hinge. This is called the turning effect of force. Similarly, we use turning effect when we open or close a water tap.

If the distance between two points of a body remains the same under the action of a force, it is called a rigid body. During rotation, all the particles of the rigid body rotate along fixed circles. The straight line joining the centres of these circles is called the axis of rotation. In this case, it is OZ.

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Activity 4.1 Take your class to the playground where a see-saw is available. Let a lighter child sit on the left side and a heavier one on the right side of the see-saw. The distances of both children from the pivot should be equal. The force exerted by each child is equal to his weight acting downward. Does the heavier child move down? Yes, because he is exerting a larger force. Now move the heavier child nearer to the pivot and the lighter child away from the pivot. You will see that the see-saw tilts to the opposite direction and the lighter child moves down. This shows that the turning effect of a force does not depend only on its magnitude but also on the location where it acts. Therefore, the greater the force, the greater is its turning effect. Moreover, the larger the perpendicular distance of the force from the axis of rotation, the greater is its turning effect.

The line along which the force acts is called the line of action of the force.

The perpendicular distance of the line of action of a force from the axis of rotation is known as the moment arm of the force.

It is harder to open a door by pushing it at a point closer to the hinge as compared to pushing it at the handle. That is why door or window handles are always installed at larger distances from hinges. Similarly, it requires greater force to open a nut by a spanner if you hold it closer to point A than point B.