# PSEB 9th Class Science Notes Chapter 9 Force and Laws of Motion

This PSEB 9th Class Science Notes Chapter 9 Force and Laws of Motion will help you in revision during exams.

## PSEB 9th Class Science Notes Chapter 9 Force and Laws of Motion

→ Gallileo and Isac Newton put forth the scientific basis with regard to the motion of the objects.

→ The concept of force is based on the activity of pull, push or hit/kick.

→ By the application of force, the size and shape of an object can be changed.

→ Force is of two types:

• Balanced force
• Unbalanced force

→ An unbalanced force acting on an object produces motion in the object.

→ The frictional force acts against the direction of motion of an object.

→ The first law of motion is also called the Laws of Inertia.

→ The tendency of the body to remain in the state of rest or of uniform motion is called Inertia.

→ Everybody opposes the change in its state of motion.

→ The moment of inertia of the train is more than that of the cart so that it does not get displaced on being pushed. In other words, the heavier bodies have more inertia.

→ The inertia of an object is the measure of its mass.

→ The momentum ‘p’ of an object is measured by the product of its mass ‘m’ and velocity ‘v’. i.e. p = m × v

→ The momentum has both magnitude and velocity. Its direction is the same as that of velocity.

→ The S.I. unit of momentum is kg ms.

→ Force changes the momentum of an object.

→ To reduce the effect of friction the surface is either made plane or the surface is painted with a lubricant.

→ S.I. unit of force is the newton.

→ For the motion of the object due to the force acting on it, Newton gave three fundamental laws of motion which hold good for all types of motion.

→ According to the first law, everybody tends to remain in its state of rest or of uniform motion unless some external force acts on it.

→ According to the second law, the rate of change of momentum of a body is directly proportional to the unbalanced force acting on it and in its direction.

→ According to the third law, when one body applies force on another body, the second body also applies instantaneous force on the first body. These two forces are equal in magnitude but opposite in direction.

→ By applying force the motion can be given to a body at rest and the body in motion can be brought to rest or a change can be brought about in the same direction.

→ Force: It is an external cause that produces or tends to produce a change in its state of rest or of uniform motion.

→ One Newton Force: It is the force that produces an acceleration of 1 ms-2 in a body of mass 1 kg. It is denoted by ‘N’.

→ Balanced Forces: If a number of forces acting on an object do not produce any change in its state then these forces are called Balanced Forces. Their net result is zero force.

→ Unbalanced Forces: If the result of various forces acting on an object is not zero, then these forces are called unbalanced forces.

→ Frictional Force: It is the opposing force that comes into play when a body moves over the surface of another body.

→ Inertia: It is the property of the bodies due to which they cannot change their position of rest or of uniform motion unless some external force is applied.

→ The inertia of Rest: It is the property by virtue of which a body at rest will continue to remain at rest unless some external force is applied to bring about that change.

→ The inertia of Motion: It is the property of a body by virtue of which the body in motion, will continue to move with the same uniform speed unless some external force is applied to bring about that change.

→ Momentum: The product of mass and velocity possessed by a body is called its momentum.

→ Law of Conservation of Momentum: If no external force acts on a system of particles then the total momentum of the system remains conserved.

→ Newton’s Second Law of Motion: Tine rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force.

→ Newton’s Third Law of Motion: Every action has an equal and opposite reaction.

→ Newton’s First Law of Motion: If a body is at rest, it will continue to remain at rest. And if a body is moving with a uniform velocity in a straight line it will continue to do so unless some external force is applied to bring about that change in its state.