International System of Units (SI)
SI is the modern from of the metric system and is the world’s most widely used system of
measurement.
Base units are the building blocks of SI – all other units of measurement can be derived from the
base units.
SI base Units are as follows :
Derived Units
Derived Units are formed by powers, products or quotients of the base units and are unlimited
in numbers.
Some Important Units of Long Distance
Light Year = Distance travelled by light in vaccum in one year = 9.46 * 10^12 k m.
1 Parsec (Parallactic second ) is the distance corresponding to a parrallex of one second of arc , 1
Parsec = 3.085 * 10 m. 1 Parsec = 3.26 light years.
Some Important Units of Short Distances
1 micron (1 u)=10 cm, 10 m
1 Angstrom (1 A) = 10 cm, 10 m
1 Fermi (1F)= 10 cm, 10 m
Scalar Quantities
Physical quantities which have magnitude only and no direction are called scalar quantities.
Example: Mass, Speed, Work, Volume, Time, Power, Energy, etc.
Vector Quantities
Physical quantities which have magnitude and direction both and which obey the law of
triangle are called vector quantities. Example: Displacement, Velocity, Acceleration, Force,
Momentum, Torque, etc.
Distance
Distance is length of actual path covered by a moving object in a given time interval.
Distance is a scalar quantity.
Displacement
Shortest distance covered by a body in a definite direction is called displacement.
In general, magnitude of displacement< distance.
Speed
Distance travelled by a moving object in unit interval of time is called speed. It us a scalar
quantity and its SI unit is meter/second (m/s) i.e
Velocity
Velocity is the rate of change of the position of an object, equivalent to a specification of its
speed and direction of motion. Velocity is a vector physical quantity; both magnitude and
direction are required to define it. The scalar absolute value (magnitude) of velocity is called
“speed”. It does not keep track of direction; velocity is a vector quantity and is direction aware.
Angular Velocity
The angle subtended by the line joining the object from the origin of circle in unit time interval
is called angular velocity.
Acceleration
It is defined as the rate of change of velocity of the object i.e
Acceleration = Change in velocity- Time It is a vector quantity and its unit is meter/second (m/s)
If velocity decreases with time, then acceleration becomes negative and is called retardation or
deceleration.
Circular Motion
If an object describes a circular path (circle), then its motion is called circular motion. If object
moves with a uniform circular speed, then its motion is called uniform circular motion.
Uniform Circular Motion is an accelerated motion because the direction of velocity changes
continuously.
Newton’s laws of Motion
Newton, the Father of Physics has established the laws of motion in his book “Principia” in 1667.
Newton’s First Law of Motion
An object maintains its initial position of rest or motion with uniform speed on a straight line
unless an external force acts on it.
First law is also called the law of Galilieo or law of inertia.
Inertia : Inertia is the property of a body by virtue of which the body opposes change an initial
state of rest or motion with uniform speed on a straight line.
Inertia is of two types namely
(i) Inertia of rest
(ii) Inertia of motion.
First law gives the definition of force.
Force:
Force is that external cause which when acts on a body changes or tries to change the
initial state of the body.
Momentum :
Momentum is the property of a moving body and is defined as the product of
mass and velocity of the body i.e
Momentum = mass*velocity
It is a vector quantity having SI unit kg-m/s.
Newton’s Second Law of Motion
The rate of change in momentum of a body is directly proportional to applied force on the
body and it takes place in the direction of the force.
If F= applied force, a= acceleration produced and m = mass of the body
then, F=ma.
Newton’s second law gives the magnitude of force.
Newton’s first law is contained in the second law.
Newton’s Third Law of Motion
To every action, there is an opposite and equal reaction. Examples of third law: Recoil of gun,
Motion of a rocket, Swimming etc.
Conservation of Linear Momentum:
It follows from Newton’s second and third law that if no external force acts on a system of
bodies, the total linear momentum of the system of bodies remains constant.
As a consequence, the total momentum of bodies after and before collision remains the same
This is called the principle of conservation of momentum.
Impulse
When a large force acts on a body for a very small time, then force is called impulsive force.
Impulse is defined as product of force and time.
Impulse = force * time = change in momentum
It is a vector quantity and its direction is direction of force. Its SI unit is Newton second (Ns).
Force
A force is a push or pull upon an object resulting from the object’s interaction with another
object.
Centripetal Force
When a body travels along a circular path (circle), its velocity changes continuously, naturally an
external force always acts on the body towards the centre of the path.
The external force required to maintain the circular motion of the body is called centripetal
force.
If a body of mass is moving on a circular path of radius R with uniform speed v, then the
required centripetal force
Centrifugal Force
In applying Newton’s laws of motion, we have to consider some forces which cannot be
assigned to any object in the surroundings. These forces are called pseudo force or inertial
force.
Centrifugal force is such a pseudo force. It is equal and opposite to centripetal force.
Cream separator, centrifugal driver works on the principle of centrifugal force.
Centrifugal force should not be confused as the reaction to the centripetal force because forces
of action and reaction act on different bodies.
Moment of Forces
The rotational effect of a force on a body about an axis of rotation is described in terms of
moment of force.
Moment of force about an axis of rotation is measured as the product of magnitude of force and
the perpendicular distance of direction i.e Moment of force= Force*Perpendicular distance from
the fixed axis.
It is a vector quantity.
Its unit is Newton meter (Nm).
Centre of Gravity
The centre of gravity of the body is that point through which entire weight of body acts.
The centre of gravity of a body does not change with the change in the orientation of the body
in the space.
The weight of the body acts through centre of gravity in the downward direction. Hence a body
can be brought to equilibrium by applying a force equal to its weight in the vertically upwards
direction through centre of gravity.
Equilibrium
If the resultant of all the forces acting on a body is zero, then the body is said to be in
equilibrium.
If a body is in equilibrium, then it will be either in the state of rest or in uniform motion. If it is at
rest, the equilibrium is called static otherwise dynamic.
Static equilibrium is of following three types :
Stable Equilibrium : if on slight displacement from equilibrium position, a body regains its
original position, it is said to be in stable equilibrium.
Unstable Equilibrium : If on slight displacement from equilibrium position , a body moves in the
direction of displacement and does not returns to its original position, the equilibrium is said to
be unstable equilibrium is said to be unstable equilibrium. In this equilibrium, the centre of
gravity of the body is at the highest position.
Neutral Equilibrium : If on slight displacement from equilibrium position, a body has no
tendency to come back to the original position or to move in the direction of displacement, it is
said to be in neutral equilibrium, the centre of gravity always remains at the same height in such
equilibrium.
Friction
Friction is a force that is cre
Friction always opposes the motion or attempted motion of one surface across another surface.
Friction is dependent on the texture of both surface.
Friction is also dependent on the amount of contact force pushing the two sur
(normal force).
Advantage of Friction
Due to friction we are able to walk on the surface of the Earth.
The brakes applied in automobiles
Nails, screws and the wooden boarders are held together due to force of
The fibers of thread are held together due to force of friction.
Disadvantages of Friction
In overcoming the friction, a lot of energy is wasted in the form of heat. Friction causes wear
and tear of the moving parts.
Due to friction, speed of au
Reducing Friction: Friction cannot be eliminated.
Otherwise, friction can be reduced to a great extent by using certain materials as follows :
Rollers
Oil, wax, grease
Talcum powder
Air cushion
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