Class 9 Physics | Punjab Curriculum and Textbook Board Syllabus 2025
If the parallel forces are acting in the same direction, then they are called like parallel forces, and if their directions are opposite to one another, they are called unlike parallel forces.
If the distance between two points of the body remains the same under the action of a force, it is called a rigid body.
During rotation, all the particles of the rigid body rotate along fixed circles. The straight line joining the centres of these circles is called the axis of rotation.
The line along which the force acts is called the line of action of the force.
The perpendicular distance of the line of action of a force from the axis of rotation is known as moment arm of the force, or simply moment arm.
The turning effect of a force is the force that causes a rigid body to rotate about an axis.
Moment of a force or torque is defined as the product of the force and the moment arm. The magnitude of torque is given by:
\[ \tau = F \times l \]
Where τ (tau) is the torque, F is the applied force, and l is the moment arm. The SI unit of torque is newton metre (Nm).
When two equal and opposite parallel forces act at two different points of the same body, they form a couple. A couple is a special type of torque.
The process of dividing a force into its components is known as the resolution of a force.
Typically, a force is resolved into two components that are perpendicular to each other. These components are called the perpendicular or rectangular components of the force.
Trigonometry is a branch of mathematics that deals with the properties of a right-angled triangle.
The principle of moments is defined as: When a body is in balanced position, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point.
Centre of gravity is that point where total weight of the body appears to be acting.
The centre of mass of a body is that point where the whole mass of the body is assumed to be concentrated.
A body is said to be in equilibrium if it has no acceleration.
A body at rest is said to be in static equilibrium.
A body moving with uniform velocity is said to be in dynamic equilibrium.
A body is said to be in translational equilibrium only if the vector sum of all the external forces acting on it is equal to zero. Mathematically,
\[ \sum F = 0 \]
The vector sum of all the torques acting on a body about any point must be zero. Mathematically,
\[ \sum \tau = 0 \]
A body is said to be in a state of stable equilibrium, if after a slight tilt, it comes back to its original position.
A body is said to be in a state of unstable equilibrium if, after a slight tilt, it tends to move further away from its original position.
A body is in neutral equilibrium if it comes to rest in its new position after disturbance without any change in its centre of mass.
The force that causes an object to move in a circle at constant speed is called the centripetal force. Mathematically,
\[ F_c = \frac{mv^2}{r} \]