# 9th Class Physics Chapter 4 Notes

Parallel forces have their lines of action parallel to each other.

If the direction of parallel forces is the same, they are called like parallel forces. If two parallel forces are in opposite direction to each other, then they are called unlike parallel forces.

The sum of two or more forces is called the resultant force.

A graphical method used to find the resultant of two or more forces is called head to tail rule.

Splitting up a force into two components perpendicular to each other is called resolution of that force. These components are

Centre of mass of a body is such a point where a net force causes it to move without rotation.

The centre of gravity of a body is a point where the whole weight of a body acts vertically downward.

A couple is formed by two parallel forces of the same magnitude but acting in opposite direction along different lines of action.

A body is in equilibrium if net force acting on it is zero. A body in equilibrium either remains at rest or moves with a uniform velocity.

A body is said to satisfy second condition for equilibrium if the resultant torque acting on it is zero.

A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position.

If a body does not return to its previous position when sets free after slightly tilt is said to be in unstable equilibrium.

A body that remains in its new position when disturbed from  its previous   position is said   to be in a state of neutral equilibrium.

A force can be determined from its perpendicular components as

Torque or moment of a force is the turning effect of the force. Torque of a force is equal to the product of force and moment arm of the force.

According to the principle of moments, the sum of clockwise moments acting on a body in equilibrium is equal to the sum of anticlockwise moments acting on it.