Every day, we perform many activities such as walking, lifting a school bag, pushing a trolley, riding a bicycle, or switching on a fan. All these activities involve work and energy. Work is done when a force causes an object to move, while energy is the ability to do work. Understanding these concepts helps us explain how objects move and how machines perform tasks.
Work Done by a Constant Force:
In science, work is said to be done when a force applied on an object causes it to move in the direction of the force.
For work to be done, two conditions must be satisfied:
- A force must act on the object.
- The object must move (show displacement).
Work done by a constant force is the product of the force applied and the displacement produced in the direction of the force.
Examples of Work Done:
- Pushing a trolley and making it move.
- Pulling a suitcase across the floor.
- Lifting a bucket of water from a well.
- Kicking a football.
In all these cases, force causes movement, so work is done.
Examples Where Work is Not Done:
- Pushing a wall that does not move.
- Holding a school bag while standing still.
- Sitting on a chair.
In these cases, displacement is zero, so no work is done.
Important Point
Force alone cannot produce work. Displacement is also necessary.
Mathematical Expression for Work Done:
Work done can be calculated using the formula:
\( W=F\times s\)
Where:
- \(W\ =\) Work done
- \(F\ =\) Force applied
- \(s\ =\) Displacement in the direction of the force
SI Unit of Work
The SI unit of work is Joule (\(J\)).
One joule of work is done when a force of \(1\ newton\) moves an object by \(1\ metre\) in the direction of the force.
Positive, Negative, and Zero Work
Positive Work
Positive work is done when force and displacement are in the same direction.
Examples:
- Pulling a cart forward.
- Pushing a box across the floor.
- A horse pulling a carriage.
The force helps the object move.
Negative Work
Negative work is done when force acts opposite to the direction of displacement.
Examples:
- Friction acting on a moving object.
- Brakes slowing down a bicycle.
- Air resistance acting on a moving vehicle.
The force opposes motion and reduces the object's energy.
Zero Work
Zero work is done when:
- Displacement is zero, or
- Force acts perpendicular to displacement.
- Pushing a wall that does not move.
- Holding a suitcase while standing.
- Carrying a bag while walking on a level road.
Different Forms of Energy
| Forms of energy | Meaning and examples |
| Kinetic Energy |
Kinetic energy is the energy possessed by an object due to its motion.
Examples
The faster an object moves, the greater its kinetic energy.
|
| Potential Energy |
Potential energy is the energy stored in an object due to its position or shape.
Examples:
Potential energy can change into kinetic energy when the object starts moving.
|
| Chemical Energy |
Chemical energy is the energy stored in substances.
Examples
When these substances react, chemical energy is released.
|
| Heat Energy |
Heat energy is the energy associated with warmth or temperature.
Examples
|
| Light Energy |
Light energy helps us see objects around us.
Examples
|
| Sound Energy |
Sound energy is produced by vibrating objects.
Examples
|
| Electrical Energy |
Electrical energy is produced by the movement of electric charges.
Examples
|
| Solar Energy |
Solar energy is the energy obtained from the Sun.
Examples
Solar energy is renewable and environmentally friendly.
|
Interconversion of Energy
Energy can neither be created nor destroyed. It can only be converted from one form to another.
This process is called energy interconversion.
| Situation | Energy Conversion |
| Electric bulb | \(Electrical\ \to\ Light\ +\ Heat\) |
| Electric fan | \(Electrical\ \to\ Mechanical\) |
| Candle burning | \(Chemical\ \to\ Heat\ +\ Light\) |
| Solar panel | \(Solar\ \to\ Electrical\) |
| Photosynthesis |
\(Solar\ \to\ Chemical\)
|
| Battery-operated toy car | \(Chemical\ \to\ Mechanical\) |
| Water flowing from a dam | \(Potential\ \to\ Kinetic\) |
| Human body during running | \(Chemical\ \to\ Mechanical\) |
Energy interconversion in daily life