# 9th Class Physics Chapter 3 Important Short Questions 2024

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## 9th Class Physics Chapter 3 Short Questions Notes

• A force is a push or pull. It moves or tends to move, stops or tends to stop the motion of a body.
• The inertia of a body is its property due to which it resists any change in its state of rest or uniform motion in a straight line.
• The momentum of a body is the quantity of motion possessed by the body. The momentum of a body is equal to the product of its mass and velocity.
• The force that opposes the motion of a body is called friction.
• Newton’s first law of motion states that a body continues its state of rest or of uniform motion in a straight line provided no net force acts on it.
• Newton’s second law of motion states that when a net force acts on a body, it produces acceleration in the body in the direction of the net force.
• The magnitude of this acceleration is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, F = ma.
• SI unit of force is Newton (N). It is defined as the force that produces 2 an acceleration of 1 ms” in a body of mass 1 kg.
• The mass of a body is the quantity of matter possessed by it. It is a scalar quantity. SI unit of mass is kilogram (kg).
• The weight of a body is the force of gravity acting on it. It is a vector quantity. SI unit of weight is Newton (N).
• Newton’s third law of motion states that to every action there is always an equal and opposite reaction.
• The acceleration and tension in a system of two bodies attached to the ends of a string that passes over a frictionless pulley such that both move vertically.
• The acceleration and tension in a system of two bodies attached to the ends of a string that passes over a frictionless pulley such that one moves vertically and the other parts of the machine. The friction can be reduced by:
• (i) Smoothing the sliding surfaces in contact.
• (ii) Using lubricants between sliding surfaces.
• (iii) Using ball bearings or roller bearings.
• The law of conservation of momentum states that the momentum of an isolated system of two or more two interacting bodies remains constant.
• A force between the sliding objects which opposes the relative motion between them is called friction.
• Rolling friction is the force of friction between a rolling body and a surface over which it rolls. Rolling friction is lesser than the sliding friction.
• The friction causes loss of energy in machines and much work has to be done in overcoming it. Moreover, friction leads to much wear and tear on the moving.
• The motion of a body moving along a circular path is called circular motion.
• The force that keeps the body moving in a circular path is called the centripetal force and is given.
• According to Newton’s third law of motion, there exists a reaction to the centripetal force. The centripetal reaction that pulls the string outward is sometimes called the centrifugal force.