General Science : Laws of Motion & Applications

By Asha Gupta|Updated : December 31st, 2021

Complete coverage of syllabus is a very important aspect for any competitive examination but before that important subject and their concept must be covered thoroughly. In this article, we are going to discuss General Science: Laws of Motion & Applications

Laws of Motion & Applications

INERTIA

The property of an object by virtue of which it cannot change its state of rest or of uniform motion along a straight line it's own is called inertia.

Inertia is a measure of the mass of a body. Greater the mass of a body greater will be its inertia or vice-versa.

Inertia is of three types:

  • The inertia of Rest- When a bus or train starts to move suddenly, the passengers sitting in it falls back due to the inertia of rest.
  • The inertia of Motion- When a moving bus or train stops suddenly, the passengers sitting in it jerks in the forwarding direction due to inertia of motion.
  • The inertia of Direction- We can protect ourselves from the rain with an umbrella because raindrops can not change their direction its own due to inertia of direction.

Newton’s First Law of Motion

A body continues to be in its state of rest or in uniform motion along a straight line unless an external force is applied to it.

This law is also called the law of inertia.

Examples

  • When a carpet or a blanket is beaten with a stick then the dust particles separate out from
  • If a moving vehicle suddenly stops then the passengers inside the vehicle bend

Newton’s Second Law of Motion

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.

Mathematically F &infi; DP / DTF = k (d / DT) (mv) where k is a constant of proportionality and its value is one in SI and CGS system.

F= mdv / dt = ma

Newton’s Third Law of Motion

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For every action, there is an equal and opposite reaction and both acts on two different bodies

Mathematically F12 = – F21 Examples

  • Swimming becomes possible because of the third law of
  • Jumping of a man from a boat onto the bank of a
  • Jerk is produced in a gun when the bullet is fired from (iv) Pulling of the cart by a horse.

Note Newton’s second law of motion is called the real law of motion because the first and third laws of

motion can be obtained by it.

Law of Conservation of Linear Momentum

If no external force acts on a system, then its total linear momentum remains conserved.

Linear momentum depends on the frame of reference but the law of conservation of linear momentum is independent of the frame of reference.

Newton’s laws of motion are valid only in an inertial frame of reference.

Weight (w)

It is a field force, the 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. w = mg

Rocket

Rocket is an example of variable mass following the law of conservation of momentum. Friction

A force acting on the point of contact of the objects, which opposes the relative motion is called friction.

It acts parallel to the contact surfaces.

Frictional forces are produced due to intermolecular interactions acting between the molecules of the bodies in contact. Friction is of three types:

  1. Static Friction

It is an opposing force that comes into play when one body tends to move over the surface of the other body but the actual motion is not taking place.

Static friction is a self-adjusting force that increases as the applied force is increased,

  1. Limiting Friction

It is the maximum value of static friction when the body is on the verge of starting motion. Limiting friction (fs) = μsR

Where,

μs,   =       coefficient       of       limiting friction R = normal reaction.

Limiting friction does not depend on the area of contact surfaces but depends on their nature, i.e., smoothness or roughness.

If the angle of friction is θ, then the coefficient of limiting friction μs = tan θ

  1. Kinetic Friction

If the body begins to slide on the surface, the magnitude of the frictional force rapidly decreases to a constant value fk kinetic friction. Kinetic friction, FK = μk N where μ k = coefficient of kinetic friction and N = normal force.

Kinetic friction is of two types:

  • Sliding friction
  • Rolling friction

As, rolling friction < sliding friction, therefore it is easier to roll a body than to slide. Kinetic friction (FK) = μk R where μk = coefficient of kinetic friction and R = normal reaction.

The angle of repose or angle of sliding It is the minimum angle of inclination of a plane with the horizontal, such that a body placed on it, just begins to slide down. If the angle of repose is a. and the coefficient of limiting friction is μ, then μs = tan α

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