Comprehensive study notes for Work and Energy (Chapter ) Physics Matric 9th. Read detailed explanations, solve MCQs, practice questions with answers. Free online education Pakistan.
Work: Work is done when a force acting on a body displaces it in the direction of the force.
Formula: $W = F \times S$
If the force makes an angle $\theta$ with the direction of motion, then:
$W = F S \cos\theta$
The SI unit of work is Joule (J).
Definition of Joule: The amount of work is one joule when a force of one newton displaces a body through one meter in the direction of the force.
$1 J = 1 N \times 1 m = 1 Nm$.
Definition: The energy possessed by a body due to its motion is called Kinetic Energy.
Formula: $K.E = \frac{1}{2}mv^2$
Derivation: Consider a body of mass $m$ moving with velocity $v$. A force $F$ stops it after distance $S$.
Work done = Change in K.E = $F \times S$.
Using $2aS = v_f^2 - v_i^2$ (where $v_i=v, v_f=0, a=-F/m$):
$2(-F/m)S = 0 - v^2 \implies 2FS/m = v^2 \implies FS = \frac{1}{2}mv^2$.
Definition: The energy possessed by a body due to its position is called Potential Energy.
Formula: $P.E = mgh$
Derivation: To lift a body of mass $m$ to height $h$, force required is $F=mg$.
Work done = $F \times h = (mg) \times h = mgh$.
This work is stored as Gravitational Potential Energy.
Statement: Energy cannot be created nor destroyed but can be converted from one form to another. The total amount of energy remains constant.
Consider a body of mass $m$ at height $h$. It has $P.E = mgh$ and $K.E = 0$.
As it falls, P.E decreases and K.E increases.
Just before hitting the ground, P.E becomes 0 and K.E becomes maximum ($mgh$).
Thus, loss in P.E = gain in K.E.
Sources that take millions of years to form and cannot be replaced once used.
- Fossil Fuels: Coal, Oil, Gas (release $CO_2$, cause pollution).
- Nuclear Fuels: Uranium (releases huge heat via fission).
Sources that can be replenished or reused.
- Solar Energy: From Sun (Solar panels, cells).
- Wind Energy: Wind turbines.
- Hydroelectric: From dams (falling water).
- Geothermal: Heat from Earth's interior (Magma).
- Biomass: Organic waste (garbage, crops).
Definition: The ratio of useful output energy to the total input energy is called efficiency.
Formula: $Efficiency = \frac{Output}{Input} \times 100\%$
Ideal System: A system with 100% efficiency. Practical systems always have efficiency < 100% due to friction and heat loss.
Power: The rate of doing work is called Power.
Formula: $P = \frac{Work}{Time} = \frac{W}{t}$
Unit: SI unit is Watt (W).
$1 Watt = 1 Joule / 1 Second$.
Another unit is Horsepower ($1 hp = 746 W$).