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# CBSE Class 9 Physics Notes for Chapter 2 Force and Laws of Motion

CHAPTER 2 FORCE AND LAWS OF MOTION

An unbalanced force is required to set an object in motion.

LAWS OF MOTION

There are three laws of motion:

• The first law states that an object remains in a state of rest or of uniform motion in a straight line unless an external force acts on it. The natural tendency of an object to resist a change in its state of motion or rest is called inertia.

• The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Mathematical formulation of the law gives:

F=ma, where m=mass, a= acceleration and F=Force

Momentum (p): It is the product of its mass and velocity i.e. p=mv

• The third law of motion states that every action has an equal and opposite reaction acting on two different bodies.

In an isolated system, the total momentum remains conserved. This implies that during collision:    m1u1 + m2u2 = m1v1 + m2v2