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# TNPSC Free Notes Physics In English – Laws of motion

இந்தக் கட்டுரையில், TNPSC குரூப் 1, குரூப் 2, குரூப் 2A, குரூப் 4 மாநிலப் போட்டித் தேர்வுகளான TNUSRB, TRB, TET, TNEB போன்றவற்றுக்கான  முறைகள் இலவசக் குறிப்புகளைப் பெறுவீர்கள்.தேர்வுக்கு தயாராவோர் இங்குள்ள பாடக்குறிப்புகளை படித்து பயன்பெற வாழ்த்துகிறோம்.

### Laws of motion

Newton’s First Law:
Every object continues to be in the state of rest or of uniform motion (constant velocity) unless
there is external force acting on it.
Inertia:
Inertia is that property of a body by virtue of which the body is unable to change its state by itself
in the absence of external force.
Inertia at rest:
 It is the inability of the body to change its state of rest by itself.
 A person standing in a bus falls backward when the bus suddenly starts moving. This is
because, the person who is initially at rest continues to be at rest even after the bus has
started moving.
Inertia at motion:
 Inertia of motion is the inability of the body to change its state of motion by itself.
 When a passenger gets down from a moving bus, he falls down in the direction of the
motion of the bus.
Inertia at direction:
It is the inability of the body to change its direction of motion by itself. When a bus moving
along a straight line takes a turn to the right; the passengers are thrown towards left.
Newton’s Second Law:
 The force acting on an object is equal to the rate of change of its momentum(p)
 p = m x v (mass x velocity)
 One Newton is defined as the force which acts on 1 kg of mass to give an acceleration 1
m s -2 in the direction of the force.
 F = m x a

Newton’s Third Law:
Newton’s third law states that for every action there is an equal and opposite reaction.

Units of force:
 SI unit of force is newton (N) and in C.G.S system its unit is dyne.
 1 N = 1 kg m s –2
 1 N = 10 5 dyne.
Newton’s universal law of Gravitation:
 Every particle of matter in this universe attracts every other particle with a force.
 This force is directly proportional to the product of their masses and inversely
proportional to the square of the distance between the centers of these masses.

 The direction of the force acts along the line joining the masses.
 F =(G x m1 x m2) / r 2
 G = 6.674 × 10 -11 N m 2 kg –2
Weight and Mass:
Mass:
 Mass of a body is defined as the quantity of matter contained in the body.
 Its SI unit is kilogram (kg).
Weight:
 Weight of a body is defined as the gravitational force exerted on a body due to the
gravity.
 Weight = Gravitational Force = mass (m)× acceleration due to gravity(g).

 g = acceleration due to gravity for Earth (at sea level) = 9.8 ms –2 .
Impulse:
 If a very large force acts on an object for a very short duration, then the force is called
impulsive force or impulse.
 Unit is Ns.
Force:
Distance
 Distance is the actual path length travelled by an object in the given interval of time
during the motion
 Unit : metre in SI system.
 Quantity : scalar
Displacement
 Displacement is the difference between the final and initial positions of the object in a
given interval of time
 Unit : metre in SI system.
 Quantity : vector

Speed:
 Speed = Distance travelled / Time
 Quantity = Scalar
 S.I Unit = m/s
Velocity:
 Velocity = Displacement / Time
 Quantity = vector

 S.I Unit = m/s
Acceleration:
 Acceleration = Speed or Velocity / Time
 S.I Unit = m/s 2
 A = (Final velocity-Initial velocity)/ Time
 A = (v–u)/t

Equations of Motions:
 v = u + at
 s = ut + ½ at 2
 v 2 = u 2 + 2as
o Initial velocity = u
o Final velocity = v
o Time = t
o Acceleration = a and
o Distance = s.
Motion of a freely falling body:
 Replacing ‘a’ in equations with g, the acceleration due to gravity.
 For a freely falling body which is initially at rest

 u = 0
 v = gt,
 s = gt 2
 v 2 = 2gh
Uniform circular motion:
 Speed = Circumference/Time taken
 V = 2πr/T

Centripetal force:
For circular motion, a constant force should act on the body,along the radius towards the centre
and perpendicular to the velocityof the body. This force is known as centripetal force.
Centrifugal force:
The equal and opposite reaction to the centripetal force is called centrifugal reaction or force,
because it tends to take the body away from the centre.
Work:
 Work is said to be done, when a force acts on a body and the point of application of the
force is displaced in the direction of force.
 W = F x S
 The SI unit is joule.
 One joule of work is said to be done when a force of one newton acting on a body
displaces it by one metre.
Power:
 Power (P) is defined as ‘the rate of doing work’. It can also be defined as ‘the work done
per unit time’.
 Power (P) is calculated by dividing the work done (W) by the time taken (t) to do that
work.
 Power = work done / time taken
 Power is said to be one watt when one joule of work is done in one second
 One watt of power is the same as one joule per second
Energy:
 Energy is defined as the capacity to do work.
 The SI unit for measuring energy is the same as that of measuring work, which is the
joule the practical unit of measuring electrical energy is the kilowatt-hour
 This is equivalent to 3600000J [1000W x 3600s = 3600000J = 3.6 x 106 J].
The Law of Conservation of Energy:
Energy can neither be created nor destroyed; it can only be changed from one form to another.’
Types of energy:
 Mechanical energy

 Electrical energy
 Heat energy
 Light energy
 Sound energy
Mechanical energy:
The energy acquired by objects upon which work is done is known as mechanical energy
Kinetic energy:
 Energy possessed by an object due to its motion (or velocity) is called kinetic energy.
 KE = ½ mv 2
 m = Mass of the object
 v = velocity
Potential energy:
 The energy possessed by a body by virtue of its position or due to a state of strain, is
called potential energy.
 PE = mgh,
 m = mass of the object
 g = Acceleration due to gravity
 h = Height

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