Physics: Inertia, Force, Newton’s Laws of Motions,  Weight and Friction

This article discusses important concepts related to Inertia, Force, Newton’s Laws of Motions,  Law of Conservation of Linear Momentum, weight and friction concepts for General Knowledge of Physics for SSC and UPSC Prelims Examinations.


The property of an object by virtue of which it cannot change its state of rest or of uniform motion along a straight line its own, is called inertia. Inertia is basically a measure of mass of the body. Thus, greater is the mass, greater is its inertia and vice versa.

  • When a bus or train starts to move suddenly, the passengers sitting in it falls backward. This is due to inertia called inertia of rest.
  • When a moving train stops suddenly, the passengers sitting in it jerk in forward direction. This is due to inertia called inertia of motion.
  • We are able to protect ourselves from rains using an umbrella because rain drops cannot change their direction on their own. This is called inertia of direction.


Force refers to a push or pulls which tries to change the state of rest, motion, size or shape of an object. Its SI unit is Newton. 1 Newton is equal to 1 kg m/s2. There are two types of forces viz. Contact forces and Action at distance forces. Examples of Contact Forces include Frictional force, Tensional Force, Spring Force etc. The Forces in action at distance include magnetic force, electrostatic force, gravitational force etc.

Further, the forces which act on an object for a short interval of time but change large change in momentum is called impulsive force. Momentum is the total amount of motion present in a body. Change in Momentum is called Impulse.

Newton’s First Law of Motion

Newton’s first law of motion says that a body continues to be in its state of rest or in uniform motion along a straight line unless an external force is applied on it. This explains:

  • Why when a beat a carpet with stick, dust particles separate out of it.
  • Why passengers feel sudden jerk forward when a moving Bus or train stops suddenly.

Newton’s Second Law of Motion

Newton’s second law of motion says that the rate of change of linear momentum is proportional to the applied force and change in momentum takes place in the direction of applied force. This explains:

  • Why it is easier to push empty cart than full cart
  • Why adult is able to push or pull a cart easily than a child

The second law of motion is called real law of motion because first and third laws of motion can be obtained by it.

Newton’s Third Law of Motion

Third law of motion says that “For every action there is an equal and opposite reaction and both acts on two different bodies.” Swimming is possible because of this law. This explains why jerk is produced in a boat when bullet is fired from it. A person is hurt on kicking a stone due to reaction only.

Law of Conservation of Linear Momentum

This law says that if no external force acts on a system, then its total linear momentum remains conserved. In equation form, Momentum=mass*velocity. To increase the momentum of an object, we need to either increase its mass or velocity or both.

  • Rockets work on law of conservation of momentum. As momentum in one direction is given to the rocket’s exhaust gases, momentum in the other direction is given to the rocket itself.

Weight (w)

Weight refers to a force with which a body is pulled towards the centre of the earth due to gravity. It has the magnitude mg, where m is the mass of the body and g is the acceleration due to gravity, thus w=mg

  • When a lift is either at rest or moving with a constant speed, then apparent weight of a person standing in it is equal to his actual weight. Thus, R = mg
  • When a lift is accelerating upward, then apparent weight would be R1=M(g+a). Thus weighing machine would read the apparent weight more than the actual weight.
  • When a lift is accelerating downward, then apparent weight would be R2=m(g-a). Thus, the weighing machine would read less than actual weight.
  • When the lift is falling freely under gravity then apparent weight R3=m(g-g) =0. In this case, machine will read zero.
  • If lift is accelerating downward with an acceleration greater than g, then the person will lift from floor to the ceiling of the lift.


Friction is force acting on the point of contact of the objects, and which opposes the relative motion. Friction always works parallel to the contact surfaces. Frictional forces are produced due to intermolecular interactions acting between the molecules of the bodies in contact.

There are three kinds of friction viz. static friction, limiting friction and Kinetic friction.

Static friction is the opposing force which works when one body tends to move over the surface of the other body but actual motion is not taking place. This makes harder for two objects to slide alongside one another. Glass on Glass is an example of static friction. Static friction results from the interlocking irregularities present on the two surfaces in contact. This force will increase in response to an attempt to move the objects until it is overcome at the threshold of motion. The maximum value of static friction when body is at the verge of starting motion is called Limiting Friction. The friction that occurs after the point where motion is achieved is referred to as kinetic friction.

Common examples of Friction:
  • We can hold a pen while writing due to the force of friction. Friction is needed in this case for better grip. If there is no friction, it would be really difficult to write.
  • If there was no friction, walking on the road would become impossible. It is friction that allows us to walk.
  • After a shower, it becomes difficult to drive a car at high speed on the wet road because friction decreases.
  • Angle of sliding or angle of repose is the minimum angle of inclination of a plane with the horizontal in such a way that the body placed on it begins to slide down. It depends upon limiting friction.
  • Further, when a body moves on an inclined plane then several forces work on it viz. normal reaction of plane, friction force acting in opposite direction of motion, gravitation force vertically down etc.

Pushing or pulling an object

To pull an object (such as lawn mower) is always easier than to push whenever the force is applied at an angle to the object. This is because horizontal component of force will act to move the object. so:

  • If we push, then the vertical component of force will press the object downward and the friction will be more.
  • If we pull then the Vertical component of force will act upward and the friction will be less.

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