Laws Of Motion Class 11 Notes Physics Chapter 5 - CBSE

Chapter : 5

What Are Laws Of Motion ?

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    It is defined as a push or pull which changes or tends to change the state of rest of uniform motion or the direction of motion of the body.


    It is the inherent property of a material body by virtue of which it cannot change by itself, its state of rest or of uniform motion.


    It is the quantity of motion in a body. It is given by p = mv, its SI unit is kg ms–1.

    Newton’s Law Of Motion

    First Law

    Newton’s first law of motion states that every body continues in its state of rest or of uniform motion along a straight line, unless an external force is applied to change that state.

    Second Law

    Newton’s second law states that the rate of change of momentum of a body is directly proportional to the applied force.

    $$\text{F}=\frac{\text{dp}}{\text{dt}} =\frac{\text{d}}{\text{dt}}(\text{mv})\\= m\frac{\text{dt}}{\text{dt}}$$

    Third Law

    Newton’s third law states that to every action,
    there is an equal and opposite reaction.

    $$\vec{\text{F}}_{\text{BA}} = -\vec{\text{F}}_{\text{AB}}$$


    It is the total effect of a large force which acts for a short time to produce a finite charge in mometum.

    Impulse = Force × Time duraction

    Apparent Weight Of Body In A Lift

    • When a lift moves upwards with unifor acceleration, then apparent weight

    R = m(g + a)

    • When lift moves downwards with acceleration a, then apparent weight

    R = m(g – a)

    • During free fall (a = g)

    R = m(g – g) = 0

    Law Of Conservation Of Linear Momentum

    It states that in the absence of any external force, vector sum of the linear momentum of a system of particles remains constant.


    When a body moves or tends to move over the surface of another another body, a force comes into play which acts parallel to the surface of contact and opposes the relativemotion. This opposing force is called friction.

    • The force of friction which comes into play between two bodies before one body actually starts moving over the other is called static friction.
    • The maximum force of static friction which comes into play when a body just starts moving over the surface
      of another body in called limiting friction.
    • The force of friction which comes into play when a body is in a steady motion over the surface of another
      body is called kinetic or dynamic friction (fc). Kinetic friction is less than limiting friction.
    • Coefficient of limiting friction is the ratio of limiting friction to the normal friction.
    • Centripetal Force: It is the force required to make a body move along a circular path with a uniform speed.
      It always acts along the radius and towards the centre of the circular path.

    $$\text{F} =\frac{\text{mv}^{2}}{\text{r}}=mr\omega^{2}\\=mr(2\pi v)^{2}\\ = mr\bigg(\frac{2\pi}{\text{T}}\bigg)^{2}$$

    • Centrifugal force is a fictitious force acting radially outwards on a particle moving in a circle and is equal in
      magnitude to the centripetal force.
    • If fc is the coefficient of friction between tyres and road, then the maximum velocity with which the vehicle can safely take a circular turn of radius r is given by

    $$v =\sqrt{\mu rg}$$

    • The maximum angle with which a vehicle (in the absence of friction) can negotiate a circular turn of radius r and banked at an angle θ is is given by

    $$v =\sqrt{\text{(rg tan)}\space\theta}$$

    When the frictional forces are also taken into account, the maximum rate is given by

    $$v =\sqrt{\text{rg}\bigg(\frac{\mu +\text{tan}\theta}{\text{1 +}\mu\text{tan}\theta}\bigg)}$$

    • In order to take a circular turn of radius r with speed v, the cyclist should bend himself through an angle θ from the vertical such that,

    $$\text{tan}\space\theta =\frac{v^{2}}{\text{rg}}$$