Gravity and Weight vs Mass
In 1969, Neil Armstrong weighed about 750 N on Earth — but on the Moon he weighed only 125 N, even though his body contained exactly the same 76 kg of matter.
Printable Worksheets
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Q1 · An astronaut on the Moon can jump 6× higher than on Earth. Does their mass change on the Moon?
Q2 · Why do you feel lighter in a swimming pool?
● Know
- Mass = amount of matter (kg), never changes
- Weight = gravitational force (N), changes with location
- g ≈ 9.8 N/kg on Earth (each kg of mass feels 9.8 N of pull)
● Understand
- Why mass stays constant but weight varies on different planets
- The difference between a balance scale and a spring scale
- Why "weightlessness" in orbit is really free fall
● Can do
- Calculate approximate weight given mass and gravitational field strength
- Compare weights on Earth, Moon and Mars
- Explain why astronauts in the ISS feel weightless
- Mass
- Weight
- Gravity
- Gravitational field strength
- Newton
- The force of gravity pulling an object down — measured in Newtons
- The amount of matter in an object — measured in kilograms
- The unit of force, symbol N
- How strongly gravity pulls on each kilogram of mass (e.g. 9.8 N/kg on Earth)
- The non-contact force that attracts all objects with mass toward each other
is the amount of matter in an object and is measured in . is the force of gravity and is measured in .
Pick up your school bag and step onto the bathroom scales: both give a number. Now imagine stepping off a spaceship onto the Moon — the scales read one-sixth as much, yet the bag feels exactly as hard to swing. Your mass is 50 kg whether you're in Sydney, on the Moon, or in deep space — and matter doesn't disappear just because gravity changes.
Mass is the amount of matter in an object. Key facts:
- Measured in kilograms (kg) using a balance scale
- A balance scale compares your mass against known masses — it works the same anywhere in the universe
- Mass stays constant everywhere — in Sydney, on Mars, in orbit
- A greater mass means more matter (more atoms) making up the object
Common mistake: "I've lost mass on a diet." Actually, you've lost mass AND weight — but the mass changed, and the weight changed as a result. You can't change your weight without also changing your mass (on the same planet).
- Earth
- Moon
- Mars
- ~1.6 N/kg
- ~3.7 N/kg
- ~9.8 N/kg
Weight is the force of gravity pulling an object toward the centre of a planet. It is a force, so it is measured in Newtons (N), not kilograms.
The formula: W = m × g (weight = mass × gravitational field strength)
| Location | g (N/kg) | Weight of a 50 kg person |
|---|---|---|
| Earth | 9.8 | 490 N |
| Moon | 1.6 | 80 N |
| Mars | 3.7 | 185 N |
| Jupiter | 24.8 | 1240 N |
On the Moon, a 50 kg person weighs only 80 N — about one-sixth of their Earth weight. They'd feel incredibly light and could jump huge distances. But their mass is still 50 kg.
Bathroom scales actually measure weight force (how hard gravity pushes you onto the scale). They are calibrated to display mass in kg — but on the Moon, that scale would show about 8 kg even though you're still a 50 kg person. This is why scientists always distinguish mass from weight.
A 50 kg astronaut travels to Mars (g = 3.7 N/kg). Their is still 50 kg but their is now only 185 . On the Moon, they would weigh even .
A common myth: "There's no gravity in space." This is completely wrong. Gravity acts everywhere in the universe — it just gets weaker as you move further from a massive object.
The ISS (International Space Station) orbits Earth at about 400 km altitude. At that height, gravity is still about 90% as strong as on Earth's surface. So why do astronauts float?
The answer: They are in continuous free fall. The ISS and everything inside it is falling toward Earth, but it's also moving sideways so fast that it keeps "missing" the planet. That sideways motion creates an orbit. Both the astronauts and the ISS are falling at exactly the same rate — so relative to each other, they float.
Think of it this way: if you jump out of an aeroplane without a parachute (please don't), you and your bag would both fall at the same rate — your bag would appear to "float" next to you. That's what orbit is — an endless falling-and-missing.
- Gravity extends into deep space — it just gets weaker with distance
- The Moon's gravity holds it in orbit around Earth
- The Sun's gravity holds Earth (and all planets) in orbit
Astronauts in the ISS feel because they are in continuous around Earth. Gravity has not — they are just falling and missing the planet at the same time. This is called an .
An astronaut has a mass of 80 kg. They travel to the Moon (g = 1.6 N/kg) and then to Mars (g = 3.7 N/kg). Predict: (a) what is their mass on the Moon? (b) approximately what is their weight on Mars compared to on Earth (g = 9.8 N/kg)?
How close was your prediction?
Well done — mass stays constant is the key insight here.
Remember: mass never changes location to location. Weight = m × g.
Using W = m × g, calculate the weight of a 60 kg student on each planet/moon. Gravitational field strengths: Earth 9.8, Moon 1.6, Mars 3.7, Venus 8.9, Jupiter 24.8 N/kg.
| Location | g (N/kg) | Mass (kg) | Weight (N) |
|---|---|---|---|
| Earth | 9.8 | 60 | |
| Moon | 1.6 | 60 | |
| Mars | 3.7 | 60 | |
| Venus | 8.9 | 60 | |
| Jupiter | 24.8 | 60 |
Classify each item or instrument below as measuring/relating to Mass (M), Weight (W), or Both (B). Give a one-line reason.
| Item | M, W, or B? | Reason |
|---|---|---|
| A balance scale | ||
| A spring scale (Newton meter) | ||
| The reading on a bathroom scale | ||
| "My backpack is 5 kg" | ||
| "The skydiver pulled 700 N on their harness" | ||
| The gravitational pull on the Moon | ||
| "I weigh 60 kg" (casual speech) | ||
| An astronaut's body after returning from orbit |
Q1. A person has a mass of 70 kg on Earth (g ≈ 9.8 N/kg). (a) What is their weight on Earth? (b) What is their mass on the Moon? (c) What would their weight be on the Moon (g ≈ 1.6 N/kg)? (3 marks)
Q2. Explain the difference between mass and weight. Why is this distinction important? (3 marks)
Q3. Why do astronauts feel "weightless" in the International Space Station even though Earth's gravity still reaches that altitude? (3 marks)
Answers
▾MCQ 1
C — W = m × g = 60 × 9.8 = 588 N. Weight is in Newtons not kilograms (D is wrong units). 60 N would only be correct if g were 1 N/kg.
MCQ 2
C — A balance scale compares masses using known weights on both sides. It works anywhere because both sides experience the same gravity, so the ratio is always correct. A spring scale (Newton meter) measures weight force, which changes location to location.
MCQ 3
D — Mass is the amount of matter in the astronaut's body and never changes regardless of location. Her weight would be less on Mars, but not her mass.
MCQ 4
C — Both the ISS and the astronauts are in continuous free fall around Earth. Since everything falls at the same rate, there is no sensation of weight relative to the station. Gravity is still ~90% as strong at that altitude as on Earth's surface.
MCQ 5
C — Weight is a force and is measured in Newtons. Kilograms measure mass. Grams are a unit of mass. Pascals measure pressure.
Short Answer 1
Model answer: (a) W = 70 × 9.8 = 686 N. (b) Mass on Moon = 70 kg (mass never changes). (c) W = 70 × 1.6 = 112 N.
Short Answer 2
Model answer: Mass is the amount of matter in an object (measured in kg) and never changes. Weight is the gravitational force acting on that mass (measured in N) and changes with location. The distinction matters because weight depends on where you are — a 70 kg person weighs 686 N on Earth but only 112 N on the Moon, and understanding this is essential for space exploration, engineering and comparing measurements across different planets.
Short Answer 3
Model answer: Gravity at the ISS altitude (~400 km) is still about 90% as strong as on Earth's surface, so "no gravity" is a myth. Astronauts feel weightless because the ISS is in continuous free fall around Earth — it falls toward Earth but travels sideways so fast that it keeps missing the planet (orbital motion). Both the astronauts and the station fall at exactly the same rate, so relative to each other there is no sensation of being pulled down — they appear to float.
The hook at the start of this lesson asked: bathroom scales display kilograms — but they're actually measuring a force. So are they measuring mass or weight? Now you know the answer — and it's a surprisingly sneaky trick!
Explain what the bathroom scales are really measuring, and why they'd give a wrong reading if you took them to the Moon. Use the words mass, weight and gravitational field strength in your response.
- Mass (kg) = amount of matter — never changes. Weight (N) = gravitational force = m × g — changes with location.
- On Earth g ≈ 9.8 N/kg; Moon ≈ 1.6; Mars ≈ 3.7 — same mass, very different weights on each.
- Astronauts feel "weightless" in orbit because they are in continuous free fall — gravity still acts on them.