Force & Laws of Motion Notes | SSC CGL, CHSL | Railways | IB

Force & Laws of Motion Notes | SSC CGL, CHSL | Railways | IB

Force (F)

A force is any interaction that, when unopposed, will change the motion of an object.

   F = m x a
  • A force can cause an object with mass to change its velocity, i.e., to accelerate.
  • Force can also be described intuitively as a push or a pull.
  • Vector quantity
  • SI unit: N
  • CGS Unit: dyne
  • 1N = 1 Kg m s-2
  • 1N = 105 dyne
  • Balanced and Unbalanced Forces: Balanced forces do not cause a change in motion whereas Unbalanced Forces does.

Fundamental Forces (In order of Increasing Strength)

1. Gravitational Force (Gravity)

It is a natural phenomenon by which all things with mass or energy, including planets, stars, galaxies, and even light are brought toward one another.

  • Very long range

2. Weak Nuclear Force

These forces were discovered during the study of the phenomenon of β-decay, in radioactivity.

  • The WNF is 1025 times stronger than Gravitational Force.
  • Very short range

3. Electromagnetic Force

Force which acts between charged particles and is the combination of all electrical and magnetic forces.

  • Can be attractive or repulsive
  • Electrostatic forces: When the charges are at rest.
  • These forces are governed by coulomb’s law.
  • Long range

4. Strong Nuclear Force

The forces that bind the neutrons and protons together in a nucleus.

  • Very short range

Gravitational Forces are the weakest and strong nuclear forces are the strongest forces among the four fundamental forces.


The natural tendency of objects to resist a change in their state of rest or uniform motion is called inertia.

  • Inertia of Rest
  • Inertia of Motion
  • Inertia of Direction

Sir Isaac Newton (4 January 1643 – 31 March 1727)

  • Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”) – a book by Newton, first published in 1687, laid the foundations of classical mechanics.

Newton’s First Law of Motion (AKA Law of Inertia)

An object continues to be in a state of rest or of uniform motion along a straight line unless acted upon by an unbalanced force.

   Ft = mv - mu 
   (when F = 0, v = u 
   i.e., initial velocity = final velocity)

Applications around Us

  • Only the carom coin at the bottom of a pile is removed when a fast moving carom coin (or striker) hits it.
  • When the playing card is flicked with the finger, the coin placed over it falls in the tumbler.
  • Tree leaves and fruits gets detached from the three, if we vigorously shake its branch.
  • Passengers fall in forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest.

Momentum (p)

A property of a moving body that the body has by virtue of its mass and motion.

   p = m x v
  • Has the same direction as that of the velocity.
  • Vector quantity
  • SI Unit: kg m s-1

Newton’s Second Law of Motion

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

   F ∝ (p2 - p1) / t
     ∝ (mv - mu) / t
     ∝ [m x (v - u)] / t
     ∝ ma                (∵ (v - u) / t = a)
   F = kma
   F = ma                (∵ for 1 unit of force, k =  1)

Newton’s Third Law of Motion (AKA Law of Action and Reaction)

To every action, there is an equal and opposite reaction and they act on two different bodies.


Applications around Us

  • A forward force on the bullet and recoil of the gun.
  • As the sailor jumps in forward direction, the boat moves backwards.

Law of Conservation of Momentum

In an isolated system, the total momentum remains conserved.

   Fa + Fb + Fc + . . . + Fz = 0


If a large force is acting on a body for a very short time, then the product of this large force and time is known as impulsive force.

   Impulse = change in momentum
   Impulse = f x t
  • Vector Quantity
  • SI Unit: N s


  • Hitting a ball with bat, firing a gun


  • A cricket player lowers has hands while catching the ball : by doing so the time of impact increases and hence the effect of force decreases.
  • When a person falls from a certain height on floor, he receives more injuries as compared to falling on a heap of sand. It is because the Cemented floor does not yield whereas the sand yield there by increasing the time of impact hence decreasing the impact of force.
  • The shock absorbers provided in the vehicle helps to travel smoothly on an uneven road. It is because the shockers increases the time of impulse which reduces the force.

Apparent weight of a person in a lift

lift at rest

apparent weight = actual weight

moving uniformly in upward or downward direction

apparent weight = actual weight

accelerating upwards

apparent weight > actual weight

accelerating downwards

apparent weight < actual weight

lift is under free fall

apparent weight = 0 (feel of weightlessness)


When a body slides or rolls over another body or on a surface, then a force opposing the motion acts between those surfaces of the body which are in contact. This force is called as force of friction.

  • Types of friction: Rolling Friction, Fluid Friction, Sliding Friction, etc.

Friction can be reduced by

  • Changing sliding friction into rolling friction
  • By using lubricants such as oil, water, or grease

Advantages of Friction

  • It is becomes difficult to walk on a slippery road due to low friction. When we move on ice, it becomes difficult to walk due to low friction of ice.
  • We can not fix nail in the wood or wall if there is no friction. It is friction which holds the nail.
  • A horse can not pull a cart unless friction furnishes him a secure Foothold.
  • We can write on a paper or on a board.
  • Friction helps in applying the brakes.

Disadvantages of Friction

  • The main disadvantage of friction is that it produces heat in various parts of machines. In this way some useful energy is wasted as heat energy.
  • Due to friction, noise is also produced in machines.
  • It counteracts movement and so it takes more energy to move.
  • Forest fires are caused due to friction between branches of trees

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