Forces And Laws Of Motion Class 9 Notes Science - Chapter 9

Chapter: 9

What Are Forces And Laws Of Motion ?

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    • A force is that physical cause which changes (or tends to change) either the size or the shape or the state of rest or of motion of the body
    • Force is vector quantity as it has both magnitude as well as direction.
    • The SI unit of force is Newton.

    Types of Force

    Contact Non Contact
    • The forces which act on bodies when they are in physical contact are called contact force.
    • The forces experienced by bodies even without being physically touched are called non-contact forces or forces at a distance.
    Example: Frictional force Example: Gravitational Force

    Action and reaction forces are always equal and opposite. According to Newton’s third law, there is an equal (in size) and opposite (in direction) reaction force for every action force. Forces always come in pairs known as action-reaction force pairs.


    The change in position of an object with respect to time is said to be motion. Flying of bird, movement of train, running of child etc. are some examples of motion.

    Laws of motion

    • Newton’s first law of motion (law of inertia): According to Newton’s first law of motion, an object will either remain at rest or move in uniform motion on a straight line unless acted upon by an external force which can change its state of rest or motion. Every object in this universe resists the change in its state.
    • Newton’s second law of motion: According to Newton’s second law of motion, the rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force.
    • Newton’s third law of Motion: According to Newton’s third law of motion, to every action, there is an equal and opposite reaction. In other words, when a body exerts a force on another body, the second body instantly exerts a force of equal magnitude to the applied force back onto the first body. These forces are always opposite in direction and act on two different objects always.

    Mass And Inertia

    • Mass is a measure of inertia of the body. Greater the mass, greater is the inertia, and thus, more is the resistance to the change in the state of rest or motion.

    Kinds of inertia is of two kinds:

    • Inertia of rest: If a body is at rest, then it will continue to remain at rest unless an external force is applied.
    • Inertia of motion: If a body is in a state of motion, then it will continue to move at the same speed in the same direction unless an external force is applied to change its state.

    Inertia: The property of an object by virtue of which it neither changes its state nor tends to change the state is called inertia.