Ch 3 · 1.3 Mass and Weight · O Level Physics 5054

Topic 3 of 17 in Motion, Forces and Energy — O Level Physics 5054 — 69 lessons.

Introduction

An astronaut on the Moon bounces along easily, yet a push to start a heavy trolley there is just as hard as on Earth. That is because weight changes from place to place but mass does not. This lesson separates the two, shows how each is measured, and links them through the gravitational field strength. (Syllabus section 1.3.)

Key ideas — Mass

Mass is a measure of the quantity of matter in an object at rest relative to the observer. It is measured in kilograms (kg) and it is a scalar. Mass does not change when an object is taken to the Moon or into space.

🧑‍🏫 Teacher's explanation

Imagine you are trying to push a heavy wooden crate across a smooth floor. Even before you move it, you feel a 'stubbornness'—the crate doesn't want to start moving, and once it is moving, it is hard to stop. This 'stubbornness' is mass. Mass isn't just how much 'stuff' is inside an object; it is a measure of how much that object resists being moved or stopped.

📖 Definitions

🔍 Worked example — Easy

A block of wood has a mass of 0.5 kg. Calculate its weight on Earth where g = 10 N/kg.

🔍 Worked example — Standard

An object weighs 60 N on a planet where the gravitational field strength is 12 N/kg. Determine the mass of the object.

🔍 Worked example — Exam-style

An astronaut has a mass of 75 kg on Earth. On the Moon, the gravitational field strength is 1.6 N/kg. Calculate the astronaut's weight on the Moon.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Inertia

The mass of an object resists any change in its state of rest or motion. This property is called inertia. A loaded supermarket trolley is harder to start, and harder to stop, than an empty one, because it has more mass.

🧑‍🏫 Teacher's explanation

Imagine you are sitting in a car that suddenly accelerates. Your body jerks backward. This isn't because a force pushed you backward, but because your body wants to keep doing exactly what it was doing: staying still. This 'stubbornness' is what we call inertia.

📖 Definitions

🔍 Worked example — Easy

An object has a mass of 5.0 kg. Calculate its weight on Earth where g = 10 m/s^2.

🔍 Worked example — Standard

A force meter shows a reading of 25 N when a mass is suspended from it. Calculate the mass of the object. Assume g = 10 m/s^2.

🔍 Worked example — Exam-style

An astronaut on a planet experiences a weight of 300 N. If the mass of the astronaut is 75 kg, calculate the gravitational field strength g on that planet.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Weight

Weight is the gravitational force on an object that has mass. It is a force, so it is measured in newtons (N), it is a vector, and it acts towards the centre of the Earth (downwards).

🧑‍🏫 Teacher's explanation

Imagine you are trying to push a heavy shopping trolley versus an empty one. Even if you push them both with the same force, the heavy one is much harder to get moving and harder to stop. This 'stubbornness' is what we call mass. Mass isn't about how much space something takes up, but how much 'stuff' (matter) is inside it.

📖 Definitions

🔍 Worked example — Easy

A textbook has a mass of 1.5 kg. Calculate its weight on Earth (take g = 10 N/kg).

🔍 Worked example — Standard

An astronaut weighs 750 N on Earth. Calculate their mass.

🔍 Worked example — Exam-style

A probe is sent to a planet where the gravitational field strength is 3.7 N/kg. If the probe has a mass of 500 kg, calculate its weight on this planet.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Gravitational field

A gravitational field is a region in which a mass experiences a force due to gravitational attraction. The Earth, the Moon and every other mass are surrounded by one.

🧑‍🏫 Teacher's explanation

Imagine you are standing in a room. If there is a giant magnet hidden under the floor, you would feel a pull towards it. A gravitational field is like that 'invisible zone' of pull. It is a region of space where any object placed inside it will feel a force pulling it toward the center of the mass creating the field.

📖 Definitions

🔍 Worked example — Easy

An object has a mass of 5.0 kg. Calculate its weight on Earth where g = 10 m/s^2.

🔍 Worked example — Standard

A force meter shows that a 2.5 kg block weighs 24.5 N on a certain planet. Calculate the gravitational field strength g on this planet.

🔍 Worked example — Exam-style

An astronaut's mass is 75 kg on Earth. On the Moon, the gravitational field strength is 1.6 N/kg. Calculate the astronaut's weight on the Moon.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the mass of the sphere. [1] (b) Calculate the weight of the sphere on Earth (take g = 10 m/s^2). [2] [3 marks]

Key ideas — Gravitational field strength

The gravitational field strength g is the force per unit mass:

🧑‍🏫 Teacher's explanation

Imagine you are pushing a shopping trolley. If the trolley is empty, it's easy to start it moving or stop it. If the trolley is full of heavy groceries, it's much harder to change its speed. This 'stubbornness' to change motion is what we call mass. Mass isn't how heavy you feel; it's how much 'stuff' is inside you.

📖 Definitions

🔍 Worked example — Easy

An object has a mass of 5.0 kg. Calculate its weight on Earth, where g = 9.8 N/kg.

🔍 Worked example — Standard

An astronaut weighs 150 N on a planet where the gravitational field strength is 3.75 N/kg. Calculate the mass of the astronaut.

🔍 Worked example — Exam-style

A probe is sent to a moon where it weighs 24 N. If the probe's mass is 15 kg, calculate the gravitational field strength g on this moon.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Measuring mass and weight

Because both pans are in the same gravitational field, the balance compares weights, and therefore masses. It gives the same result on the Moon.

🧑‍🏫 Teacher's explanation

Imagine you are trying to push a heavy shopping trolley versus a light pebble. Even if you are in deep space where you feel weightless, that heavy trolley still requires a huge push to get it moving. This 'stubbornness' to change motion is what we call mass. Mass is simply how much 'stuff' is inside an object.

📖 Definitions

🔍 Worked example — Easy

An object has a mass of 0.5 kg. Calculate its weight on Earth where g = 10 m/s^2.

🔍 Worked example — Standard

A force meter reads a weight of 14 N when a block is hanging from it. Calculate the mass of the block. Use g = 9.81 m/s^2.

🔍 Worked example — Exam-style

An astronaut's mass is 75 kg on Earth. On a distant planet, they weigh 150 N. Calculate the gravitational field strength g on that planet.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Example 1: weight on Earth and the Moon. A rover has a mass of 180 kg.

🔍 Worked example — Easy

A block of wood has a mass of 450 g. Calculate its weight on Earth, where g = 10 m/s^2.

🔍 Worked example — Standard

An object weighs 54 N on a planet where the gravitational field strength is 6.0 N/kg. Calculate the mass of the object.

🔍 Worked example — Exam-style

An astronaut's mass is 75 kg. On Mars, the force meter shows their weight is 285 N. Calculate the gravitational field strength on Mars.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) 0.50 × 9.8 = 4.9\ N. (2) m = 490 / 9.8 = 50\ kg. (3) A region in which a mass experiences a force due to gravitational attraction. (4) Starting it depends on its mass (inertia), and its mass is the same on the Moon. (5) g = 15 / 4.0 = 3.75 ≈ 3.8\ N/kg; the acceleration of free fall is about 3.8\ m/s^2.

Ch 3 · 1.3 Mass and Weight · O Level Physics 5054

All chapters in Motion, Forces and Energy