Class 9 Physics | Punjab Curriculum and Textbook Board Syllabus 2025
Work is said to be done when a force acts on an object and moves it through some distance.
Mathematically, Work = Magnitude of force × Distance
\[ W = F \times S \]
One joule work is done when a force of one newton acting on a body moves it through a distance of one metre in its own direction. From equation W=FS:
\[ 1J = 1 N \times 1 m \]
\[ 1J = 1N m \]
Bigger units of work are also used like 1 kJ=103 J and 1 MJ= 106 J.
When a force acts at an angle θ with the direction of motion:
\[ W = FS \cos\theta \]
Energy is the ability of a body to do work.
When one joule of work is done on a body, the amount of energy spent is one joule.
The combination of kinetic energy and potential energy is called mechanical energy. Forms: kinetic energy and potential energy.
Kinetic energy of a body is the energy that a body possesses by virtue of its motion.
\[ E_k = \frac{1}{2} mv^2 \]
Potential energy is defined as the energy that a body possesses by virtue of its position or deformation.
\[ E_p = mgh \]
The energy possessed by an object due to its position relative to the Earth. For example, water stored at a height in a dam.
The energy stored in a compressed or stretched spring or elastic material. This energy is due to the deformation of the object.
The energy stored in chemicals, such as in batteries or fuels. It is released during chemical reactions and may be converted into electrical or thermal energy.
This is the energy released by burning fossil fuels like coal, oil, or gas through chemical reactions.
The energy stored in the nucleus of an atom. When the nucleus splits (nuclear fission), a large amount of energy is released in the form of heat and radiation.
Energy cannot be created or destroyed. It may be transformed from one form to another, but the total amount of energy never changes. Example: A falling body converts potential energy (Ep=mgh) to kinetic energy (Ek=½mv2), but total energy remains mgh.
The resources of energy which are replaced by new ones after their use are called renewable energy sources.
Non-renewable sources are those, which are depleted with the continuous use.
Fossil fuel energy is the energy that is released by burning of oil, coal and natural gas.
Hydroelectric generation is the electricity generated from the power of falling water.
Sun is the biggest source of energy. The energy obtained from sunlight is referred to as solar energy.
Solar cells are also known as photo voltaic cells. The voltage produced by a single voltaic cell is very low.
In order to get sufficient high voltage for practical use, a large number of such cells are connected in series to form a solar cell panel.
The energy released by breaking the nucleus of an atom is known as nuclear energy.
Geothermal energy is the heat energy of the hot rocks present deep under the surface of the Earth.
Wind energy is the electrical energy produced by using the kinetic energy of the fast-blowing wind.
Tidal energy is a renewable energy source that uses the movement of ocean tides (caused by the moon's gravity) to generate electricity.
Wave energy uses ocean waves (made by wind and tides) to make electricity.
It is that energy which is obtained from the biomass.
Power is defined as the time rate of doing work.
\[ P = \frac{W}{t} \]
It tells us how fast or slow work is done. Power of any agency can also be defined as energy transferred per unit time.
SI unit of power is watt (W). One watt is the work done at the rate of one joule per second.
\[ 1 W = \frac{1 J}{1 s} \quad \text{or} \quad 1 Js^{-1} \]
Bigger units of power are kilowatt (kW), megawatt (MW) etc.
\[ 1 \text{ kilo watt (kW)} = 10^3 W = 1000 W \]
\[ 1 \text{ mega watt (MW)} = 10^6 W = 1000 000 W \]
In British engineering system, the unit of power used is horse-power (hp). The horse power is defined as
\[ 1 \text{ horse power (1hp)} = 746 W \]
The ratio of useful output energy and the total input energy is called the efficiency of a working system. Thus
\[ \text{Efficiency} = \frac{\text{Useful output energy}}{\text{Total input energy}} \]
Efficiency is often multiplied by 100 to give percentage efficiency. Thus,
\[ \text{Percentage Efficiency} = \frac{\text{Useful output energy}}{\text{Total input energy}} \times 100 \]
A perpetual machine cannot work because some energy is always lost as heat due to friction and air resistance.