Force — Part 1: Turning effect of a force: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Push, pull — or turn?

Part 1 — a force can move a body along, or make it turn about a fixed point. The turning effect depends on where the force acts.

In this lesson you will

Distinguish translational and rotational motion
Calculate the moment of a force in N m and in dyne cm
Explain why handles are placed far from the axis of rotation
Find the resultant moment of several forces with a sign convention

Table of everyday actions and whether the force translates or rotates the body
Everyday action	Effect of the force
Pushing a trolley	Moves it along (translation)
Opening a door	Turns it about the hinges (rotation)
Tightening a nut	Turns it about its axis
A rolling ball	Both translation and rotation

Concept

Translational and rotational motion

In translational motion every particle of the body moves through the same distance in the same direction (a car on a straight road). In rotational motion the body turns about a fixed axis, and each particle moves in a circle about that axis (a fan blade, a door). A force applied to a body pivoted at a point produces rotation; a force on a free body through its centre of gravity produces translation.

Table comparing translational and rotational motion with examples
Motion	What happens	Example
Translational	All points move the same distance in the same direction	A lift going up
Rotational	Body turns about a fixed axis	A ceiling fan
Both	Turns while moving along	A rolling wheel

Diagram

Moment of a force

The turning effect of a force is called its moment (or torque): $\text{moment} = F \times d$ , where $d$ is the perpendicular distance from the axis of rotation to the line of action of the force. SI unit: newton metre (N m); cgs unit: dyne centimetre (dyne cm); $1\ \text{N m} = 10^{5}\ \text{dyne} \times 10^{2}\ \text{cm} = 10^{7}\ \text{dyne cm}$ . A larger moment comes from a larger force or a larger perpendicular distance.

Cookies