4.8 Equilibrium
We have learnt how translatory and rotational motion can be caused due to the application of external forces. Now, we shall see how external forces can be balanced to produce no translational or rotational effects.
If a number of forces act on a body such that their resultant is zero, the body remains at rest or continues to move with uniform velocity if already in motion. This state of the body is known as equilibrium.
lightbulbA body is said to be in equilibrium if it has no acceleration.
There are two types of equilibrium:
- Static equilibrium
- Dynamic equilibrium
A body at rest is in static equilibrium, whereas a body moving with uniform velocity is in dynamic equilibrium.
An example of static equilibrium is a book lying on a table. Only two forces are acting on it: its weight w = mg acting downward, and the normal force Fₙ that the table exerts upward on the book. Since the book is at rest, it has zero acceleration. Therefore, the sum of all forces acting on the book should be zero:
Fₙ − w = 0
Fₙ = w
Forces can act on a body without accelerating it, provided these forces balance each other.
An electric bulb hanging from a ceiling, a man holding a box, and a beam held horizontal against a wall with the help of a rope are all examples of static equilibrium.
A good example of dynamic equilibrium is a paratrooper. In a few seconds after the free fall, the parachute opens and a little later the paratrooper starts descending with uniform velocity. In this state, the force of gravity acting vertically downward on the paratrooper is balanced by the resistance of air on the parachute acting upward.