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Send in your comments and suggestions from our Comments Page 		Newton's Laws of Motion

Newtons Laws are the basic of Mechanics.

First Law
  • Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled by an external force to change that state.

    Second Law
  • The rate of change of momentum of a body is directly proportional to the eternal force acting upon it and takes place in the direction of the force.

    Third Law
  • To every action, there is always an equal and opposite reaction. Importance of First law :
    1. It explains the property of Inertia.
    2. It defines the Force.


    Inertia

    The property of a body by which it cannot by itself change its state of rest or of its uniform motion in a straight line is called Inertia.

    Units : gram (and) kilogram

    Force

    Force is that which changes or tends to change the state of rest of a body o of its uniform motion in a straight line.

    Absolute Units

    C.G.S : Dyne

    1 Dyne = 1 gm * 1 cm/sec2

    M.K.S: Newton

    1 Newton = 1 kg * 1 m/sec2



    Gravitational units :

    C.G.S : gm . wt

    1 gm.wt : A force with which a body of mass 1 gm is attracted by the earth.

    1 gm.wt = g dynes.

    M.K.S : kg . wt

    1 kg.wt : A force with which a body of mass 1kg is attracted by earth.

    1 kg.wt = g newtons.

    Weight of the body W = mg

    Importance of second law : It gives a measure of the force by F = ma

    Momentum :

    It is defined as the product of mass and velocity. (OR) P = mv and it is a vector quantity.

    Units of Momentum :

    C.G.S : gm – cm/s

    M.K.S : kg – m/s

    Impulse :

    It is defined as the product of the force and time. An impulsive force is a large forceacting for a short interval of time. (OR) I = Ft = mv- mu and it is a vector quantity.

    Units :

    C.G.S : dyne – sec (OR) gm – cm/s

    M.K.S : newton –sec (OR) kg – m/s

    Importance of Third law :

  • It explains that a force always accompanied by an opposite force.

  • It leads to the law of conservation of momentum.

    • Law of Conservation of Momentum :

      • When no external force acts on a system, the algebraic sum of the momenta of the bodies in the system remains constant in any direction after any number of collisions.

    m1v1 + m2v2 = m1u1 + m2u2

    i.e., total momentum after collision = total momentum before collision.

    Apparent and real weights :

      1. Let a man of real weight W = mg be in a stationary lift.Then the normal reaction(R) of the floor is equal W. This normal reaction we call it as the apparent weight and R = W = mg.

    2. If the lift moves up or don with uniform velocity, R = W = mg

    3. Let the lift move upwards acceleration a.

    In this case W1 (apparent weight) > W (real weight)

    R – W = ma.

    R = W1 = W + ma

    = mg + ma

    = m (g + a)

    4. If the lift moves down with an acceleration a , the real weight W is greater than

    the normal reaction or apparent weight,

    (R = W2)

    and W – R = ma.

    Or, R = W – ma = mg – ma

    Or, W2 = m (g – a)

    5. If the lift moves down with a = g, or in the case of a freely falling lift,

    R = mg – ma = 0

    Hence the apparent weight of a person is zero.



    Work

    Work is said to be done by a force when a body moves in the direction of force.

    W = F * s

    Units :

    C.G.S : erg

    M.K.S : joule(J)

    1 erg = 1 dyne * 1 cm

    1 Joule = 1 newton * 1 metre

    1 Joule = 107 ergs.

    Kilo Watt hour (K.W.H) :

    1 K.W.H = 36 * 105 J



    Power

    Power is defined as the time rate at which work is done is called power.

    P = work done / time (OR) W/t

    Units :

    C.G.S : erg S-1

    M.K.S : J s-1 (OR) watt (W)

    The power is said to be one watt

    Energy Friction