Introduction
Pushes or Pulls are very general names given to forces. A physicist is much more precise when describing forces acting on a body using terms like weight, frictional force, tension, normal reaction force, electrical force, magnetic force etc.
Force
A force is often defined as a "push" or a "pull". There is nothing untrue with this statement, but does not really tell you much about a force. Instead of defining force, it substitutes an antonym. A physicist prefers to define a force in terms of what it does.
Friction as a Contact Force
In real life an object which is moving over a horizontal surface does not continue with constant velocity when the accelerating force is removed. Instead the object slows down and eventually stops. The deceleration is caused by a frictional force between the object and the surface on which it travels. A frictional force always acts to oppose the motion of an object over a surface and is an example of a contact force.
Moments (or Torque)
It is easier to open the door by applying force near the outer edge away from the hinges.
Principle of Moments
If a body is in equilibrium under the action of a number of forces, then the algebraic sum of the moments of the forces about any point is equal to zero.
Couple
Two equal and opposite parallel forces acting along different lines on a body constitute a couple.
Center of Gravity
You must have tried to balance your exercise book, saucer or a piece of card board on your finger tips. It will be found that in each case, there is one and only one point at which these objects can be balanced.
Equilibrium and Stability
An object is said to be stable if it is steady and well balanced so that when it is pushed slightly it does not topple or fall off easily. Let us try to do a simple experiment or derive the conditions for stability.
Pressure
When a force is applied on a surface in a direction perpendicular to the surface, the force is called thrust.
Summary
Moment is defined as the turning effect of a force about a pivot or fulcrum.
