Laws of Motion

Newton’s laws of motion

First law [law of inertia]: An object remains in the state of rest or uniform motion till a force compels it to change that state. Tendency of objects to stay at rest or keep moving is inertia.


  1. when a bus starts motion our body is pulled backwards as it is at rest and the feet and bus are in motion.
  2. When a bus applies brakes the body moves forward as it’s in motion but the feet and the bus are in rest.
  3. When a car takes a turn our body swings as it was in a straight line motion.
  4. When we make a heap of carom coins and take a striker and hit them then the lowermost coin moves, but the remaining heap falls in place due to inertia.

Inertia is present in all objects; more the mass more is the inertia. Hence mass is a measure of inertia.

Second law: The rate of change of momentum of an object is proportional to the applied unbalanced force which is in the direction of the force.


1.      A cricketer catching a ball pulls his hand back to increase the time his hand takes to reduce the speed of ball to zero.

2.      An athlete also jumps on a cushion to increase the time his body would take to reduce its speed to zero.

3.      A karate artist breaks a slab of ice with a single blow using same principle.

4.       A small bullet can kill a man if its momentum is great, a heavy truck moving a small  speed can kill a man easily sue to it momentum.

Third law: Every action has an equal and opposite reaction.

e.g.: when a bullet is fired, the force by which it moves forward causes recoil of the gun. When the sailor jumps forward, his boat moves backwards.

Energy and types of Energy

Kinetic energy is possessed by an object by virtue of its motion.

Potential energy is possessed by an object by virtue of its position or configuration.

Sum of potential and kinetic energy is called Mechanical energy.

Power is the rate at which work is done. [Unit = watt] Commercial unit of energy is kilowatt hour, thus electrical energy used by an appliance in a month is in kilowatt hour this is also expressed in terms of “unit”.

e.g.: when we stretch a bow the potential energy that is stored in it gets transferred into kinetic when the bow is released. When an object is raised it has gravitational potential energy due to the work done in raising it.



                  Types of energy

                                                        Fig 1: Types of energy


In 1790 French came up with a standard system of measurements. High speed winds are accompanied by reduced air pressure.

Time period [time to complete one oscillation] of a pendulum of a given length is constant. Pendulum were discovered by Galileo Galilee

Speedometer – measures vehicle speed.

Odometer- measures distance traveled.

Galileo Galilee: proposed a new design to the telescope. Designed barometer and pendulum. Discovered moons of Jupiter and the sunspots. He said all planets revolve around the sun not earth.

Kepler's Laws:

  1. First law: Orbit of planets is ellipse with Sun as a foci.

  2. Second law: Line joining the planets and the sun sweeps equal areas in equal intervals of time.

  3. Third law: Cube of the mean distance is equal to square of the time period.

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