Lesson overview
Mass, weight and density: Why does a big log float when a small stone sinks?
Weigh the same block on Earth, the Moon and Mars to tell mass from weight; then measure the density of a wooden block, a stone and cooking oil yourself, predict what floats, and test it in water.
Suggested sequence
- Lesson 1 · Mass and weight. Mass and weight: what a balance measures, newtons, and the gravitational field strength g = W / m on Earth, the Moon and Mars. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · Measure a density. Density as mass per unit volume, and how to measure it for a regular solid, an irregular solid and a liquid. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Float or sink. Floating and sinking: predicting from densities which objects float and which liquid floats on another. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 3 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Mass and weight
- State that mass is the quantity of matter in an object (kg) and weight the gravitational force on it (N) (1.3.1, 1.3.2)
- Define gravitational field strength as force per unit mass; recall and use g = W / m, equal to the acceleration of free fall (1.3.3)
- Know that weights, and therefore masses, can be compared using a balance (1.3.4)
- Describe weight as the effect of a gravitational field on a mass (1.3.5, Supplement)
Part 2: Measure a density
- Define density as mass per unit volume; recall and use ρ = m / V (1.4.1)
- Describe how to find the density of a liquid, a regular solid and an irregular solid that sinks (volume by displacement), with the calculations (1.4.2)
Part 3: Float or sink
- Determine from density data whether an object floats or sinks in a liquid (1.4.3)
- Determine from density data whether one liquid floats on another, given that they do not mix (1.4.4, Supplement)
Misconceptions to listen for
- “Weight is measured in kilograms.” — In everyday speech, yes. In physics weight is a force, so it is measured in newtons; kilograms measure mass.
- “Mass is how heavy something feels.” — How heavy something feels is the pull on it, its weight. Mass is the amount of matter, which stays the same on the Moon.
- “The weight is 2.4 kg.” — 2.4 kg is the mass. Weight is a force: multiply the mass by g to get newtons.
- “Divide: 2.4 ÷ 9.8 = 0.24 N.” — Each kilogram is pulled with 9.8 N, so 2.4 kg is pulled with 2.4 times as much: multiply.
- “The sugar side rises, because things weigh less on the Moon.” — Things weigh less on the Moon, but the 1 kg mass on the other pan weighs less too, by the same amount. The weights stay equal.
- “A balance cannot work without Earth’s gravity.” — The Moon has a gravitational field too. A balance needs a pull on both pans; it does not need it to be Earth’s.
- “The mass on the Moon is 75 ÷ 6 = 12.5 kg.” — The astronaut is made of the same matter on the Moon. It is the weight that becomes about a sixth.
- “The weight on the Moon is 75 × 9.8 = 735 N.” — 735 N is the weight on Earth. On the Moon use the Moon’s g, 1.6 N/kg.
- “It falls at a steady speed.” — A falling stone speeds up. Only air resistance, which we are ignoring here, could stop it gaining speed.
- “A heavier stone speeds up faster.” — Without air resistance every object falls with the same acceleration, 9.8 m/s², whatever its mass.
Scope
Mass and weight, gravitational field strength g = W / m and comparing masses on a balance (1.3.1–1.3.5), density ρ = m / V, measuring the density of a liquid, a regular solid and an irregular solid that sinks, and predicting from density data whether an object floats or one liquid floats on another (1.4.1–1.4.4). Forces and their effects are in FORCE01; pressure in liquids is in PRES01.