Ch 12 · 1.7.1 Energy · O Level Physics 5054

Topic 12 of 17 in Motion, Forces and Energy — O Level Physics 5054 — 49 lessons.

Introduction

A roller coaster climbs slowly, then races down the slope. A phone charges, then warms your hand while you use it. Energy is never used up: it moves from one store to another. This lesson names the stores, the ways energy is transferred, and the two energy equations you must use. (Syllabus section 1.7.1.)

Key ideas — Energy stores

Energy is measured in joules (J). It can be stored as:

🧑‍🏫 Teacher's explanation

Think of energy like money in a bank account. You don't 'use up' money; you just move it from your savings account (a store) to your checking account (another store) or spend it on something else (a transfer). The total amount of money in your entire financial system stays the same, even if it looks different in different places.

📖 Definitions

🔍 Worked example — Easy

A 0.5 kg ball is moving at 4 m/s. Calculate its kinetic energy.

🔍 Worked example — Standard

A 2.0 kg book is lifted from a shelf 1.5 m above the floor to a height of 3.5 m. Calculate the increase in its gravitational potential energy. (Use g = 10 m/s^2)

🔍 Worked example — Exam-style

An object of mass m is dropped from a height of 10 m onto a soft cushion. The object's kinetic energy just before impact is 490.5 J. Calculate the mass of the object. (Use g = 9.81 m/s^2)

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Energy transfers

Energy moves between stores by:

🧑‍🏫 Teacher's explanation

Think of energy like money in a bank account. You don't 'lose' money; you just move it from your savings account (one store) to your checking account (another store) or spend it on a coffee (transfer to the surroundings). In physics, energy is never 'used up' in a way that it vanishes from the universe; it just changes form or moves from one object to another.

📖 Definitions

🔍 Worked example — Easy

A 0.5 kg ball is moving at 4 m/s. Calculate its kinetic energy.

🔍 Worked example — Standard

A 2.0 kg book is lifted from the floor to a shelf 1.5 m high. Calculate the increase in its gravitational potential energy. (Use g = 10 m/s^2)

🔍 Worked example — Exam-style

A 0.8 kg skater is moving at 5 m/s at the bottom of a ramp. They skate up a ramp and reach a height of 2 m. Calculate the skater's kinetic energy at the top of the ramp, assuming no energy is lost to friction. (Use g = 10 m/s^2)

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Conservation of energy

*Energy cannot be created or destroyed; it can only be transferred from one store to another.* The total energy before a process equals the total energy after. Energy that ends up in less useful stores (often internal energy of the surroundings) is called wasted or dissipated.

🧑‍🏫 Teacher's explanation

Imagine you have a bank account. You can move money from your savings to your checking account, or spend it on a coffee, but the total amount of money you 'own' doesn't just vanish into thin air—it just changes form or location. Energy works exactly the same way. It isn't 'used up' in the sense of disappearing; it just moves from one 'store' (like a battery or a height) to another (like heat or motion).

📖 Definitions

🔍 Worked example — Easy

A 2.0 kg object is moving at a velocity of 4.0 m/s. Calculate its kinetic energy.

🔍 Worked example — Standard

A 0.5 kg book is lifted from a shelf 1.2 m above the floor to a height of 2.5 m. Calculate the increase in its gravitational potential energy. (Use g = 10 m/s^2)

🔍 Worked example — Exam-style

A 1500 kg car is traveling at 20 m/s. The driver applies the brakes and the car comes to a complete stop. Calculate the total energy transferred from the kinetic store to other stores (assuming no energy is lost to the surroundings).

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Kinetic energy

E_k = tfrac12 mv^2

Key ideas — Change in gravitational potential energy

Δ E_p = mgΔ h

🧑‍🏫 Teacher's explanation

Imagine you are lifting a heavy box from the floor to a shelf. You are doing work against gravity, and that work doesn't just vanish; it gets 'stored' in the box. This stored energy is called Gravitational Potential Energy (GPE). The higher you lift it, the more energy is stored, and the more effort it would take to drop it from that height.

📖 Definitions

🔍 Worked example — Easy

A 2.0 kg book is lifted from a table to a shelf 0.50 m higher. Calculate the increase in its gravitational potential energy. Use g = 10 N/kg.

🔍 Worked example — Standard

A climber with a mass of 75 kg climbs a cliff face to a height of 120 m. Calculate the gravitational potential energy gained. Use g = 9.81 N/kg.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Example 1: kinetic energy. A 60 kg cyclist moves at 5.0 m/s.

🔍 Worked example — Easy

A car with a mass of 1200 kg is traveling at a constant speed of 20 m/s. Calculate its kinetic energy.

🔍 Worked example — Standard

A 0.50 kg ball is dropped from a height of 3.2 m. Calculate its speed just before it hits the ground, assuming no air resistance. (Use g = 10 m/s^2 for O Level).

🔍 Worked example — Exam-style

An object is thrown upwards with a kinetic energy of 500 J. It reaches a maximum height of 15 m. Calculate the mass of the object. (Use g = 9.81 m/s^2).

⚠️ Common errors

Common mistakes

Practice

Answers: (1) (a) elastic (strain), (b) chemical, (c) internal (thermal). (2) tfrac12 × 0.20 × 15^2 = 22.5\ J ≈ 23\ J. (3) 50 × 9.8 × 4.0 = 1960\ J ≈ 2000\ J. (4) The kinetic store of the car decreases; the friction force in the brakes does work, transferring energy by heating to the internal store of the brakes and surroundings (plus some sound). (5) E_k = 2.0 × 9.8 × 5.0 = 98\ J; tfrac12 × 2.0 × v^2 = 98, so v = √(98) = 9.9\ m/s.

Ch 12 · 1.7.1 Energy · O Level Physics 5054

All chapters in Motion, Forces and Energy