Examples of moments or torque Any occasion where we apply a force with the intention of causing a rotation can be used as an example of moments or torque. Applying a single force to open a door or to pushing a pedal of a bicycle are examples of moment of a force. Twisting a tap or the handlebars on a bicycle are examples of torque due to couples.We will come across further examples of couples when we deal with simple d.c. electric motors and moving-coil galvanometers in electricity.
d F
Fig 4.2: Bicycle handlebars showing a couple
SAMPLE PROBLEM 4B
A cyclist applies a force of 8 N on the handlebars toward himself with his right hand and the same size force away from himself with his left. Each hand is 13 cm from the fulcrum.What is the torque of the couple on the handlebars?
SAMPLE ANSWER 4B Using the formula for torque of a couple, making sure all values are in their standard unit: d 2 0.13 0.26 m T Fd (8)(0.26) 2.08
The turning effect of the couple is 2.08 N m. Conditions for Equilibrium
A body is in equilibrium when it is neither accelerating nor changing its rate of rotation. In order for a body to be in equilibrium the following two laws must be satisfied.
LAWS AND PRINCIPLES
Conditions for Equilibrium ■ Law 1 The vector sum of the forces in any direction is zero. Which means: forces up equal forces down, and forces left equal forces right. ■ Law 2 The vector sum of moments about any point is zero.
Which means: the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about that point. This law is also known as the ‘Law of the lever’ or the ‘Principle of moments’.
