Ch 16 · 1.7.5 Power · O Level Physics 5054

Topic 16 of 17 in Motion, Forces and Energy — O Level Physics 5054 — 19 lessons.

Introduction

Two students climb the same stairs. They do the same work, but the one who runs up is more powerful. Power is about how fast energy is transferred. (Syllabus section 1.7.5.)

Key ideas

Power is the work done per unit time, and also the energy transferred per unit time:

Imagine you are climbing a staircase. If you run up the stairs, you reach the top much faster than if you walk slowly. Even though the total 'work' done (lifting your body weight up the height) is exactly the same in both cases, your 'power' output is much higher when you run. Power isn't about how much total effort you put in, but how quickly you can get that effort done.

📖 Definitions

🔍 Worked example — Easy

A small electric motor performs 600 J of work in 12 s. Calculate the power of the motor.

🔍 Worked example — Standard

An electric heater transfers 45,000 J of energy in 3 minutes. Calculate its power in kilowatts (kW).

🔍 Worked example — Exam-style

A crane lifts a load of 2000 N through a vertical height of 15 m in 5.0 s. Calculate the average power output of the crane.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Example 1: climbing stairs. A 50 kg student climbs 6.0 m of stairs in 8.0 s.

🔍 Worked example — Easy

A student climbs a flight of stairs with a total height of 5.0 m in 10 s. Calculate the average power developed by the student. Assume the student's mass is 60 kg and use g = 10 m/s^2.

🔍 Worked example — Standard

An electric motor transfers 4.5× 10^5 J of energy in 2.5 minutes. Calculate the power of the motor.

🔍 Worked example — Exam-style

A crane lifts a load of 2000 N to a height of 15 m in 20 s. Calculate the power of the crane. Use g = 10 m/s^2 for weight if needed, but here weight is already given as a force.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) 1200/4.0 = 300\ W. (2) 60 × 7200 = 432 000\ J ≈ 4.3 × 10^5\ J. (3) W = 300 × 9.8 × 20 = 58 800\ J; P = 58 800 / 30 = 1960\ W ≈ 2.0\ kW.

Ch 16 · 1.7.5 Power · O Level Physics 5054

Introduction

Key ideas

🧑‍🏫 Teacher's explanation

📖 Definitions

🔍 Worked example — Easy

🔍 Worked example — Standard

🔍 Worked example — Exam-style

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

🔍 Worked example — Easy

🔍 Worked example — Standard

🔍 Worked example — Exam-style

⚠️ Common errors

Common mistakes

Practice

Summary

All chapters in Motion, Forces and Energy