Summary – Chapter 8- Force and Law of Motoin-Ncert – Class 9 Science

Summary – Chapter 8- Force and Law of Motoin-Ncert – Class 9 Science

First Law of Motion (Law of Inertia):

◆Objects tend to maintain their state of motion (rest or uniform motion) unless acted upon by an external force.

◆Inertia, the natural tendency of objects to resist changes in their state of motion, is the reason behind this behavior.

◆Mass, measured in kilograms (kg), quantifies the inertia of an object. The greater the mass, the greater the inertia.

◆Friction, a force that opposes the motion of objects, is an example of an external force that can disrupt an object’s state of motion.

Second Law of Motion (Force and Acceleration):

◆The rate of change of momentum of an object is directly proportional to the magnitude of the applied unbalanced force and occurs in the direction of the force.

◆The SI unit of force is the newton (N), which is equivalent to kg·m/s². One newton of force produces an acceleration of 1 m/s² on an object with a mass of 1 kg.

◆This law mathematically relates force, mass, and acceleration through the equation F = ma, where F represents force, m represents mass, and a represents acceleration.

Momentum:

◆Momentum, denoted by the symbol p, is the product of an object’s mass and velocity. It has the same direction as the velocity and is measured in kilogram meters per second (kg·m/s).

◆Momentum is a vector quantity, meaning it has both magnitude and direction. It represents the quantity of motion possessed by an object.

Third Law of Motion (Action and Reaction):

◆Newton’s third law states that for every action, there is an equal and opposite reaction. These action and reaction forces always act on two different bodies.

◆When one object exerts a force on another object, the second object responds by exerting an equal force in the opposite direction.

◆Examples of action-reaction pairs include walking (pushing against the ground propels the person forward) and the recoil of a gun when fired (the gun exerts a forward force on the bullet, and the bullet exerts an equal and opposite force backward on the gun).

Conservation of Momentum:

◆In isolated systems where no external forces act, the total momentum before an event equals the total momentum after the event. This principle is known as the conservation of momentum.