This lesson introduces three closely related concepts in physics. Work is done when a force moves an object, energy is the ability to do that work, and power is how fast that work gets done.

1. Short Notes: Core Concepts

A. Work

• Definition: In physics, work is done only when a force causes an object to move a distance in the direction of the force. Pushing against a wall that doesn't move is effort, but it's zero work.

• Formula: Work Done (W) = Force (F) × distance (d)

• Unit: The SI unit for work is the Joule (J). (1 Joule = 1 Newton-meter)

B. Energy

• Definition: Energy is the ability to do work. If something has energy, it can apply a force that causes movement.

• Unit: The SI unit for energy is also the Joule (J).

• Mechanical Energy: This is the energy an object has due to its motion or position. It comes in two forms:

  1. Kinetic Energy (KE): The energy of motion. The faster an object moves or the more mass it has, the more KE it has.

     • Formula: KE = ½ × mass (m) × velocity² (v²)

  2. Potential Energy (PE): Stored energy.

     • Gravitational Potential Energy (GPE): Energy stored in an object due to its height above a reference point. The work done to lift an object is stored as its GPE.

       • Formula: PE = mass (m) × g × height (h) (where g ≈ 10 m/s²)

     • Elastic Potential Energy: Energy stored in a stretched or compressed object, like a rubber band or spring.

C. Energy Transformation

• Energy can change from one form to another. A key example is the conversion between PE and KE.

• Example (A swinging pendulum): At the highest point, it stops for an instant (max PE, zero KE). As it swings down, PE converts to KE. At the bottom (lowest point), it moves fastest (max KE, min PE).

D. Power

• Definition: Power is the rate at which work is done or the rate at which energy is transferred. It tells you how fast work is being done.

• Formula: Power (P) = Work Done (W) / time (t)

• Unit: The SI unit for power is the Watt (W). (1 Watt = 1 Joule per second)

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2. Tips & Tricks for the Exam

• Formula Triangles: Use formula triangles for W = Fd and P = W/t to easily find any of the three variables.

• Work = GPE Gain: Remember that the work done in lifting an object vertically is equal to the gravitational potential energy it gains. If a question asks for GPE, you can calculate mgh. If it asks for the work done to lift it, you also calculate mgh (or Weight x h).

• Units Check: Always use SI units before calculating:

  • Mass must be in kilograms (kg). (e.g., convert 500g to 0.5kg)

  • Distance/Height must be in meters (m).

  • Time must be in seconds (s).

  • Velocity must be in meters per second (m/s).

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3. Important Points & Common Exam Questions

• Work Calculation: "A boy pushes a box with a force of 50 N over a distance of 4 m. How much work has he done?"

  • Answer: W = Fd = 50 N × 4 m = 200 J.

• GPE Calculation: "Calculate the potential energy gained by a 2 kg book when it is lifted to a shelf 1.5 m high. (g = 10 m/s²)"

  • Answer: PE = mgh = 2 kg × 10 m/s² × 1.5 m = 30 J.

• KE Calculation: "What is the kinetic energy of a 800 kg car travelling at 10 m/s?"

  • Answer: KE = ½mv² = ½ × 800 kg × (10 m/s)² = 400 × 100 = 40,000 J.

• Power Calculation: "A crane lifts a 500 kg load to a height of 10 m in 20 seconds. What is the power output of the crane? (g = 10 m/s²)"

  • Answer: First, find the work done: W = mgh = 500 × 10 × 10 = 50,000 J.

  • Then, calculate power: P = W/t = 50,000 J / 20 s = 2,500 W.

• Energy Transformation Question: "Describe the energy changes of a stone that is dropped from a height."

  • Answer: At the top, it has maximum gravitational potential energy. As it falls, GPE is converted into kinetic energy. Just before it hits the ground, it has maximum kinetic energy.