Mass is how much matter an object has, in kilograms, and it is the same everywhere. Weight is the force of gravity on that mass, in newtons, and depends on where you are: weight = mass × g.
Astronauts on the Moon bounce around like they're on a trampoline. Did they lose something on the way there?
Put to work in:Gear up the winch(Engineering › Mechanisms) · Pulleys(Engineering › Mechanisms) · Lift(Engineering › Capstone projects)
"Weight" in everyday talk and "weight" in science are different. This lesson sorts that out.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: an astronaut's mass
ExploreTry it on the board and compare what happens with your prediction.
Explore: Earth, Moon, Mars
Try each place. Then change the mass. Watch the weight arrow and the line under the board: mass × g = weight.
Try it yourselfYour edits are saved in this browser.
Find the 60 kg astronaut's weight on the Moon.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: mass and weight
In your book: explain to a friend who says "I weigh 50 kilograms" what is wrong with that, and what they should say.
Mass or weight?
Measured in kilograms (kg)
Measured in newtons (N)
The same on Earth and on the Moon
A force, pulling towards the planet
How much matter an object has
Changes if g changes
A 5 kg bag on Earth (g = 9.8 N/kg). What is its weight, in N?
The same 5 kg bag on Mars (g = 3.7 N/kg). Its weight, in N?
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: a rover on Jupiter
Try it yourselfYour edits are saved in this browser.
Find the weight of a 900 kg rover on Jupiter.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: do heavy things fall faster?
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Mass (kg): how much matter. The same everywhere.
Weight (N): the force of gravity on an object. Weight = mass × g.
g is the gravitational field strength: 9.8 N/kg on Earth (about 10), 1.6 on the Moon, 3.7 on Mars, 24.8 on Jupiter.
Gravity depends on mass: the Moon has far less mass than Earth, so it pulls less (smaller g, smaller weight).
Without air, all objects fall together, whatever their mass.
For teachers
Students distinguish mass (kg) from weight (N), explain gravitational force as depending on mass, and calculate weight = mass × g in different places (AC9S7U04, node 4, a hinge node). About 25 minutes.
I can explain the difference between mass and weight, with units.
I can calculate weight from mass and g.
I can explain why weight is different on the Moon and other planets.
Before class
A concept tutorial for introducing or revising the idea; it does not replace weighing masses with a newton meter. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is mass/weight confusion, which breaks F = ma in Year 10. Insist on units every time: kg for mass, N for weight.
Board notes: g values are rounded (Earth 9.8, Moon 1.6, Mars 3.7, Jupiter 24.8 N/kg). Jupiter has no solid surface; the rover question says "if it could land".
Running the lesson
Engage: ask anyone who chose 0 kg why; 'no gravity on the Moon' is common (there is gravity, just less).
Explore: have students say aloud 'mass stays, weight changes' while switching places.
Explain: the number checks need units in discussion even though the box takes a number.
In your own words: the 'I weigh 50 kg' paragraph is the evidence.
The hard part (on purpose)
Keeping mass (kilograms, the same everywhere) apart from weight (newtons). When a student says mass changes on the Moon, ask whether any matter left the astronaut on the trip.
When students get stuck
A student says mass changes on the Moon. Ask: Did any matter leave the astronaut on the trip? Fix: No. Same matter, same mass. Only the pull of gravity (weight) changes.
A student gives weight in kilograms. Ask: Is weight a force? What unit do forces have? Fix: Newtons. Kilograms are for mass.
A student says there's no gravity in space or on the Moon. Ask: Why does the astronaut land again after jumping? Fix: The Moon's gravity pulls her back, just more weakly (1.6 N/kg).
Stretch and support
Stretch: Bathroom scales show kilograms. What are they really measuring, and why would they be wrong on the Moon?
Support: Give a two-column card: Mass (kg, matter, same everywhere) and Weight (N, force, depends on g).
Where it leads
Next: relating force, mass and motion (node 5), a class task in the unit primer. In Year 10, F = ma uses mass, not weight.
In class: newton meters and 100 g masses (about 1 N each); the Apollo 15 hammer and feather video.
What each step is for
Engage: an astronaut's mass: Students who conflate mass and weight will predict mass changes on the Moon; the board shows mass staying put while weight changes.
Explore: Earth, Moon, Mars: Switching place with mass fixed, then changing mass with place fixed, separates the two quantities that W = mg combines.
Explain: mass and weight: Defining both quantities with units, then calculating weight, makes the distinction usable in Year 10's F = ma.
Mix it up: Weight is a force, so it follows the same rule as any other force: if nothing is changing, something else balances it.
Elaborate: a rover on Jupiter: A heavier object on a stronger-gravity planet extends weight = mass × g to an unfamiliar case.
Elaborate: do heavy things fall faster?: Refuting the 'heavier falls faster' misconception uses weight and mass together: double the mass, double the pull, but double the mass to move.
Now you can
Explain the difference between mass and weight, with units.
Calculate weight from mass and g.
Explain why weight is different on the Moon and other planets.