CAPF-AC (Assistant Commandant) Exam Study Material : General Science - Units & Measurements

CAPF-AC (Assistant Commandant) Exam Study Material : General Science - Units & Measurements

General Science

Units & Measurements

  • Physics is a quantitative science, based on measurement of physical quantities. Certain physical quantities have been chosen as fundamental or base quantities (such as length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity).

  • Each base quantity is defined in terms of a certain basic, arbitrarily chosen but properly standardised reference standard called unit (such as metre, kilogram, second, ampere, kelvin, mole and candela). The units for the fundamental or base quantities are called fundamental or base units.

  • Other physical quantities, derived from the base quantities, can be expressed as a combination of the base units and are called derived units. A complete set of units, both fundamental and derived, is called a system of units.

  • The International System of Units (SI) based on seven base units is at present internationally accepted unit system and is widely used throughout the world. The SI units are used in all physical measurements, for both the base quantities and the derived quantities obtained from them. Certain derived units are expressed by means of SI units with special names (such as joule, newton, watt, etc).

  • The SI units have well defined and internationally accepted unit symbols (such as m for metre, kg for kilogram, s for second, A for ampere, N for newton etc.). Physical measurements are usually expressed for small and large quantities in scientific notation, with powers of 10. Scientific notation and the prefixes are used to simplify measurement notation and numerical computation, giving indication to the precision of the numbers.

Atomic Physics

  • An atom is the smallest particle of the element that can exist independently and retain all its chemical properties.
  • Dalton’s atomic theory, which suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory.
  • Thomson proposed that:
  • An atom consists of a positively charged sphere and the electrons are embedded in it.
  • The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
  • Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus. Rutherford’s model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model.


  • Temperature is a relative measure, or indication of hotness or coldness.
  • Heat is the form of energy transferred between two (or more) systems or a system and its surroundings by virtue of temperature difference. The SI unit of heat energy transferred is expressed in joule (J) while SI unit of temperature is kelvin (K), and °C is a commonly used unit of temperature.
  • Thermometer is a device used for measuring temperatures. The two familiar temperature scales are the Fahrenheit temperature scale and the Celsius temperature scale. The Celsius temperature (tC) and the Farenheit temperare (tF) are related by: tF = (9/5) tC + 32
  • In principle, there is no upper limit to temperature but there is a definite lower limit- the absolute zero. This limiting temperature is 273.16° below zero on the celsius scale of temperature.
  • Clinical thermometer is used to measure our body temperature. The range of this thermometer is from 35°C to 42°C. For other purposes, we use the laboratory thermometers. The range of these thermometers is usually from –10°C to 110°C. The normal temperature of the human body is 37°C.


  • To understand light you have to know that what we call light is what is visible to us.Visible light is the light that humans can see. Other animals can see different types of light. Dogs can see only shades of gray and some insects can see light from the ultraviolet part of the spectrum.
  • As far as we know, all types of light move at one speed when in a vacuum. The speed of light in a vacuum is 299,792,458 meters per second.
  • Any medium through which light can travel is an optical medium. If this medium is such that light travels with equal speed in all directions, then the medium is called a homogeneous medium. The homogeneous media through which light can pass easily, are called transperant media. The media through which light cannot pass, are called opaque media. Again the media through which light can pass partly, are called translucent media.

Magnetism & Electricity


The word magnet is derived from the name of an island in Greece calledMagnesia where magnetic ore deposits were found, as early as 600 BC. Magnetite, an iron ore, is a natural magnet. It is called lodstone.

When a bar magnet is freely suspended, it points in the north-south direction. The tip which points to the geographic north is called the north pole and the tip which points to the geographic south is called the south pole of the magnet. There is a repulsive force when north poles ( or south poles ) of two magnets are brought close together. Conversely, there is an attractive force between the north pole of one magnet and the south pole of the other.

The properties of a magnet are

  1. it attracts small piece of iron towards it.
  2. it always cmes to rest in north-south direction when suspended freely.
  3. like poles repel, unlike poles attracts each other
  4. Magnetic poles always exist in pairs.
  5. the strength of a magnet is maximum at poles located near the poends


  • Motion: In physics, motion is change of location or position of an object with respect to time. Mechanical motion is of two types, transitional ( linear ) and rotational ( spin).
  • SPEED: The speed of a moving body is the rate at which it covers distance i.e. the distance it covers per unit of time.
  • Speed: (distance travelled/ time required.) The S.I. Unit of speed is m/s.
  • VELOCITY: The distance covered by an object in a specified direction in unit time interval is called velocity. The S.I. Unit of velocity is m/s.
  • Average velocity can be calculated by dividing displacement over time.
  • The instantaneous velocity shows the velocity of an object at one point.
  • The difference betwwn speed and velocity is: Speed is the distance travelled by an object in a particular time. Velocity is the speed in a particular direction.


Properties of matters: A matter can neither be created nor it can be destroyed but it can be transformed from one state to another. Matter is made of basic building blocks commonly called elements which are 112 in number. The matter is made of only one kind of element then the smallest unit of that element is called an atom. If the matter is made of two or more different elements then the smallest unit of matter is called a molecule.

Molecule is defined as the smallest unit of matter which has independent existence and can retain complete physical and chemical properties of matters.

According to kinetic theory of matter:

  • molecules are in the state of continuous motion in all possible directions and hence they posses kinetic energy which increases with the gain of heat energy or rise in temperature,
  • the molecules always attract each other,
  • the force of attraction between the molecules decreases with the increase in intermolecular spaces


  • Sound is a form of energy and like all other energies, sound is not visible to us. It produces a sensation of hearing when it reaches our ears. Sound can not travel through vacuum.
  • Sound is produced due to vibration of different objects.The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Sound moves through a medium from the point of generation to the listener.
  • In longitudinal wave the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. This is exactly how a sound wave propagates, hence sound waves are longitudinal waves. Sound travels as successive compressions and rarefactions in the medium. In sound propagation, it is the energy of the sound that travels and not the particles of the medium.


WAVES: There are three types of waves:

1. Mechanical waves require a material medium to travel (air, water, ropes). These waves are divided into three different types.

  • Transverse waves cause the medium to move perpendicular to the direction of the wave.
  • Longitudinal waves cause the medium to move parallel to the direction of the wave.
  • Surface waves are both transverse waves and longitudinal waves mixed in one medium.

2. Electromagnetic waves do not require a medium to travel (light, radio).

3. Matter waves are produced by electrons and particles.A point of maximum positive displacement in a wave, is called crest, and a point of maximum negative displacement is called trough.

Work, Power & Energy

  • When a force acting on a body produces a change in the position of the body, work is said to be done by the force. Work done on an object is defined as the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force. The unit of work is joule: 1 joule = 1 newton × 1 metre. Work done on an object by a force would be zero if the displacement of the object is zero.
  • Power is defined as the rate of doing work. Power = (work done) / (time taken). The SI unit of power is watt. 1 W = 1 Joule/second. The unit of power is also horse power. It is the power of an agent which can work at the rate of 550 foot pounds per second or 33,000 foot pounds pwe minute. 1 horse power = 746 watts.
  • An object having capability to do work is said to possess energy. Energy has the same unit as that of work.

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Study Kit for Central Armed Police Forces(AC)

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