What this chapter is about
This chapter introduces you to the scientific meaning of work, which is different from everyday usage. In science, work is done only when a force moves an object in the direction of that force. You will learn how to calculate work and understand its unit, the joule.
Energy is the capacity to do work. You will study two main forms of mechanical energy: kinetic energy (energy of motion) and potential energy (energy due to position or height). The chapter explains how energy transforms from one form to another and introduces the law of conservation of energy, which states that energy cannot be created or destroyed, only changed in form.
Simple machines make our tasks easier by multiplying force or changing its direction. You will understand how levers, pulleys, inclined planes and other devices work, and learn about mechanical advantage — how much a machine multiplies the force we apply.
Key ideas
- Work is done when a force acts on an object and the object moves in the direction of that force. If there is no movement, or movement is perpendicular to the force, no work is done.
- Work equals force multiplied by displacement in the direction of the force: W = F × s. The SI unit of work is the joule (J), where 1 joule = 1 newton × 1 metre.
- Energy is the capacity to do work. An object that can do work possesses energy.
- Kinetic energy is the energy an object has because of its motion: KE = (1/2) × m × v².
- Potential energy is stored energy due to position or configuration. Gravitational potential energy near Earth's surface is PE = m × g × h.
- The law of conservation of energy states that the total energy of an isolated system remains constant; energy only transforms from one type to another.
- Power is the rate of doing work: P = W / t. Its SI unit is the watt (W), where 1 watt = 1 joule per second.
- Simple machines (lever, pulley, inclined plane, wheel and axle, wedge, screw) help us do work by changing the magnitude or direction of the applied force.
Formulas and facts to remember
- Work: W = F × s (force times displacement in the direction of force). Unit: joule (J).
- Kinetic energy: KE = (1/2) × m × v². Unit: joule.
- Gravitational potential energy: PE = m × g × h. Unit: joule.
- Power: P = W / t. Unit: watt (W). 1 kilowatt = 1000 watts.
- Mechanical advantage (MA) = Load / Effort. It tells how many times a machine multiplies your force.
- 1 joule of work is done when a force of 1 newton moves an object by 1 metre in its direction.
- When a freely falling object loses potential energy, it gains an equal amount of kinetic energy (ignoring air resistance).
- Commercial unit of energy: 1 kilowatt-hour (kWh) = 3.6 × 10⁶ J.
Worked examples
Example 1: Calculating work done
A boy pushes a box with a constant force of 50 N along a corridor. The box moves 8 m in the direction of the push. Find the work done by the boy.
Solution: W = F × s W = 50 N × 8 m W = 400 J
The boy does 400 joules of work on the box.
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Example 2: Finding kinetic energy
A cricket ball of mass 0.15 kg is bowled at a speed of 20 m/s. Calculate its kinetic energy.
Solution: KE = (1/2) × m × v² KE = (1/2) × 0.15 kg × (20 m/s)² KE = 0.5 × 0.15 × 400 KE = 30 J
The ball has 30 joules of kinetic energy.
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Example 3: Calculating power
A woman climbs a staircase carrying a 10 kg bag. The vertical height of the staircase is 5 m and she takes 25 seconds to climb. Find the power she develops against gravity. (Take g = 10 m/s².)
Solution: Work done against gravity = m × g × h W = 10 kg × 10 m/s² × 5 m = 500 J
Power = W / t P = 500 J / 25 s = 20 W
She develops a power of 20 watts.
Common mistakes
- Thinking work is done whenever force is applied → Work requires displacement in the direction of the force; holding a heavy bag while standing still does zero work scientifically.
- Confusing mass and weight in energy formulas → Mass (kg) is used in KE and PE formulas; weight is the force (N) due to gravity.
- Using speed without squaring it in kinetic energy → Remember KE depends on v², so doubling speed quadruples kinetic energy.
- Believing machines create extra energy → Machines only transfer or transform energy; output work can never exceed input work.
- Mixing up joule and watt → Joule measures energy or work; watt measures power (rate of doing work).
Quick revision
- Work = Force × Displacement (in the same direction). Unit: joule.
- Kinetic energy = (1/2) × mass × (speed)².
- Potential energy = mass × g × height.
- Total mechanical energy stays constant when only gravity acts (conservation of energy).
- Power = Work / Time. Unit: watt.
- Simple machines make work easier, not less; they trade force for distance.