Tag: motion of system of particles and rigid bodies

The resultant of all the forces acting the body should be equal to zero. The resultant moment of all the forces acting on the body about the point of rotation should be zero.

When a body remains in the same state of motion (translational or rotational) under the influence of the applied forces, the body is said to be dynamic equilibrium.

Stable equilibrium means that, with small deviations of the body from this state, forces or moments of forces emerge which tend to return the body to the state of equilibrium. A ball located at the bottom of a spherical deepening is in a state of stable equilibrium.

There are three states of equilibrium.

Unstable equilibrium means that, with a small deviation of the body from the equilibrium state, forces emerge which tend to increase this deviation. A ball located at the top of a spherical projection is an example of unstable equilibrium. 

If a pencil standing vertically is slightly disturbed from its position it will not come back to its original position. This type of equilibrium is called unstable equilibrium, other example of unstable equilibrium are vertically standing cylinder and funnel etc.

When a body remains in the same state of motion (translational or rotational) under the influence of the applied forces, the body is said to be dynamic equilibrium.

When the center of gravity of a body lies below point of suspension or support, the body is said to be in stable equilibrium. For example a book lying on a table is in stable equilibrium.A book lying on a horizontal surface is an example of stable equilibrium. If the book is lifted from one edge and then allowed to fall, it will come back to its original position.
Other examples of stable equilibrium are bodies lying on the floor such as chair, table etc.

The resultant moment of all the forces acting on the body about the point of rotation should be zero, i.e., the sum of the anticlockwise moments about the axis of rotation must be equal to the sum of the clockwise moments about the same axis.

It's center of gravity is neither raised nor lowered. This means that its center of gravity is at the same height as before this type of equilibrium is known as neutral equilibrium.