physics for rrb je exam

Support civil January 02, 2019 rrbje 32 comments

PHYSICS
here our team provide physics capsule as per syllabus of rrb je .

Scalar Quantities
Physical quantities which have magnitude only and no direction are called scalar quantities. e.g., Mass, speed, volume, work,
time, power, energy, etc.
Vector Quantities
Physical quantities which have both magnitude and direction and also obey triangle law are called vector quantities. e.g.,
Displacement, velocity, acceleration, force, momentum, torque etc.
Projectile Motion
 For maximum range = 45°. Therefore, along jumper takes jump at an angle of 45°.
 For maximum height 𝜃 = 90°.
 The horizontal range is the same whether the body is projected at 𝜃 or 90 − 𝜃 .
 When a body is dropped freely from the top of the tower and another body is projected horizontally from the same point, both will reach the ground at the same time.
 When two balls of different masses are projected horizontally they will reach ground
at the same time.
Newton’s Laws of Motion
First Law
“Every body retains its state of rest or state of motion, until an external force is applied
on it.” This law is also known as law of inertia or law of Galileo.
First law gives the definition of inertia. Inertia is the virtue of a body due to which
it tries to retain its state.
Second Law
 “The force applied on a body is equal to the product of mass of the body and the
acceleration produced in it F = ma.”
 The second law of motion gives the definition of force.
 A force is any influence that causes an object to undergo a certain change, either
concerning its movement, direction and geometrical structure.
 SI unit of force is Newton (N).
Third Law
 “Every action have equal and opposite reaction.” Action any reaction always act on
the different bodies.
 On firing the bullet, the gunner is pushed to backward direction.
 When the boatman is jumped from the boat, the boat is pushed back.
 In a rocket, „gases are ejected with a great speed from the rocket backwards and
rocket is pushed forwards.
 While swimming, a person pushes the water backwards (action). The water pushes
the swimmer forward with the same force (reaction).
Linear Momentum
 The product of the mass and the velocity of a body is called the linear momentum of
the body.
 It is a vector quantity. Its unit is kg-m/s.
∴ Momentum = Mass × Velocity
 A heavier body has a larger linear momentum than a lighter body moving with
the same velocity.
 In the absence of external forces, the total linear momentum of teh system
remains conserved.
Application of Conservation of Linear Momentum
 When a man jumps from a boat to the shore, the boat slightly moves away from
the shore. Rocket works on the principle of conservation of momentum.
 When a bullet is fired from a gun, the gun recoils or gives a sharp pull in backward
direction.
Impulse
 If a force acts on a body for a very short time Δt, then the product of force and time
is called the impulse.
 Impulse = Change in momentum = Force × Time interval
 Its SI unit is N-s or kg-m/s.
Concept of Impulse
A cricketer moves his hands backwards while catching a ball.
FRICTION
 It we slide or try to slide a body over a surface, the motion is resisted by a bonding
between the body and the surface. This resistance is called frictional force.
Advantages and Disadvantages of Friction
 Walking is possible due to friction.
 The transfer of motion from one part of a machine to other part through belts is
possible by friction.
 Brake works on the basis of friction.
 Friction causes wear and tear of the parts of machinery in contact. Thus their lifetime gets reduced.
Methods of Reducing Friction
By polishing, by lubrication, by proper selection of material, by using ball bearing
the friction can be reduced to same extent.
Work, Energy and Power
Work
When a body is displaced by applying a force on it, then work is said to be done. It is a
scalar quantity. Its unit is joule (J).
Energy
 It is defined as capacity of doing work. Its unit is joule in SI and erg in CGS system.
 Mechanical energy is in two forms;
kinetic energy and potential energy.
Kinetic Energy
 It is the energy possessed by a body by
virtue of its motion.
 If a body of mass m is moving with velocity v, then kinetic energy
𝐾𝐸 =1/2𝑚𝑣2 =𝑝2/2𝑚 where p is the linear momentum
 When momentum is doubled, kinetic energy becomes four times.
 Kinetic energy of air is used to run wind mills.
 Kinetic energy of running water is used to run the water mills.
 A bullet fired from a gun can pierce a target due to its kinetic energy.
 If a body is moving in horizontal circle then its kinetic energy is same at all points,
but if it is moving in vertical circle, then the kinetic energy is different at different points.
Potential Energy
 It is the energy possessed by a body by virtue of its position.
 Suppose a body is raised to a height h above the surface of the earth, then
potential energy of body – mgh.
 When a body is falling downwards, then its potential energy goes on changing to
kinetic energy.
 The potential energy of the wound spring of a clock is used to drive the hands
of the clock.
 The potential energy of water in dams is used to run turbines in order to produce
electric energy using the generators.
Law of Conservation of Energy
 According to the law of conservation of energy, “energy can neither be created nor
be destroyed but it can be transformed from one from to another.
 The sum of all kinds of energies in an isolated system remains constant at all
times.
Transformation of Energy
 In a heat engine, heat energy changes into mechanical energy.
 In the electric bulb, the electric energy is converted into light energy.
 In burning coil, oil, etc, the chemical energy changes into heat energy.
 In solar cell, solar energy changes into electrical energy.
 In playing sitar, mechanical energy changes into sound energy.
 In microphone, sound energy changes into electrical energy.
 In loud speaker, electrical energy changes into sound energy.
 In battery, chemical energy changes into mechanical energy.
 In electric motor, electrical energy changes into mechanical energy.
 In candle, chemical energy changes into light and heat energy.
POWER
 Rate of doing work by a body is called power.
i.e., 𝑃𝑜𝑤𝑒𝑟 =𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 /𝑇𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
𝑃 =𝑤/𝑡
 SI unit of power is watt (W) or joule second and it is a scalar quantity.
1 Horse Power = 746 W
1 watt second (W-s) = 1 J
1 watt hour (W-h) = 3600 J
1 kilowatt hour (kW-h) = 3.6 × 10^6𝐽
 The turning effect of a force on a body is known as the moment of the force or torque.
Torque is a vector quantity.
Simple Machines
 It is based on moment of force. Moment of force = Force × perpendicular distance of
the force from the axis of rotation.
 Lever, inclined plane, screw gauge etc are simple machine.
 Scissors, sea saw, breaks of cycle, hand pump, plass are lever of first kind.
 Nut cracker and waste carrying machine are lever of second kind.
 Tong, man‟s hand and tiller are lever of third kind.
Gravitation Force
 It is always attractive in nature.
 It is the weakest force but is a long range force.
 Mathematically it is represented as 𝐹𝐺 =𝐺𝑀𝑚/𝑟2
where, 𝐹𝐺 is gravitational force, G is gravitational constant, M is the mass of first
particle, m is the mass of second particle and r is the distance between them.
 This is called Newton‟s universal law of gravitation.
 The value of G is 6.67 × 10−11 𝑁 − 𝑚2/𝑘𝑔2
Gravity
 It is the force by which Earth attracts a body towards its centre.
 The acceleration due to gravity is the rate of increase of velocity of a body falling
freely towards the Earth.
 The value of g at the surface of Earth is 9.8 𝑚/𝑠2.
 Earth is surrounded by an atmosphere of gases due to gravity. The value of g on the
Moon is 1/6th of that on the Earth surface.
Variation in the Value of g
 When we go above the surface of the Earth, the acceleration due to gravity goes
on decreasing.
 When we go below the surface of the Earth, the acceleration due to gravity goes
on decreasing and becomes zero at the centre of the Earth.
 Decreasing the rotational motion of Earth, the value of g increase.
 When we go from the equator towards the poles, the value of g goes on increasing.
 The value of g is maximum on the surface of the Earth.
Weight of a Body in a Lift
 If lift is going up with acceleration, the apparent weight of a body is more than the
true weight. If lift is going down with acceleration, the apparent weight of a body
is less than the true weight.
SATELLITE
 The heavenly body which revolves round the planets is called satellite. Moon is a
natural satellite of Earth.
 The speed of a satellite does not depend upon the mass of the satellite.
 A satellite revolving very close to earth‟s surface has a period of revolution about 84
min and its speed is nearly 8 km/s.
 Every body inside the satellite is in a state of weightlessness. Total energy of the
satellite is negative.
 Artificial satellites are of two types
Geostationary and Polar satellites
(i) The satellite whose time period is 24 h, is called geostationary satellite. It is used to
reflect TV signals and telecast TV programs from one part of the world to another. This
satellite revolves around the Earth at a height of 36000 km. INSAT 2B and INSAT 2C
are geostationary satellites of India.
(ii) Polar satellites revolve around the earth in polar orbits at a height of approximately 800 km. The time period of these satellites is approximately 84 minute.
These satellite are used for weather forecasting in studying upper region of the
atmosphere in mapping etc.
Escape Velocity
 The minimum velocity of the body that should be given to the body to enable it to
escape away from Earth‟s gravitational field is called escape velocity.
Its value on the Earth‟s surface is 11.2 km/s.
 The value of the escape velocity of a body does not depend on its mass. Its value
on the moon surface is 2.38 km/s. So, there is no atmosphere around the moon.
 Escape velocity is 2 times the orbital velocity.
 Satellites are launched with the escape velocity as needed.

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