AM-02.03 — Mass, weight, and gravity

Course compass

Goal: understand every word, symbol, unit and operation before using the formula.

Method: concrete situation → vocabulary → step-by-step calculation → three examples → check.

Open educational resource: this content is not professional training, a diploma, certification, or authorization. Examples teach principles and do not replace engineering, testing, or qualification of a real system.

1 — A 10 kg crate does not lose mass on Mars

The crate still has a mass of 10 kg on Earth or Mars. Its weight changes because weight is a gravitational force.

Mass stays the same; weight depends on local gravity.

2 — Kilograms and newtons are different units

kg: kilograms, mass;
N: newtons, force;
N/kg: newtons per kilogram, gravitational field strength.

Near the Martian surface, beginner calculations commonly use g ≈ 3.71 N/kg.

Each kilogram experiences about 3.71 N of weight near the Martian surface.

3 — Weight formula: W = m × g

W = m × g

Read it: “W equals m times g”. Here W means weight, m mass, g gravitational field strength.

W = 10 × 3.71 = 37.1 N

Why multiply? Each of the 10 kilograms experiences about 3.71 N.

4 — Earth and Mars comparison

For a mass of 80 kg:

Mars: W ≈ 80 × 3.71 = 296.8 N
Earth: W ≈ 80 × 9.81 = 784.8 N

The mass stays 80 kg.

Important: lower weight does not remove inertia. A massive crate is still hard to accelerate and stop.

5 — What is a newton?

1 N = 1 kg × 1 m/s²

One newton gives a 1 kg mass an acceleration of 1 m/s² if it is the net force along that direction.

6 — Three examples

Tool, suited astronaut and cargo examples.

Example A — 10 kg tool

W Mars = 37.1 N

Example B — 120 kg astronaut plus suit

W Mars = 120 × 3.71 = 445.2 N

Example C — Find mass from weight

Weight = 185.5 N.

m = W ÷ g = 185.5 ÷ 3.71 = 50 kg

7 — g is not universal

It varies with the celestial body and position. Use a value appropriate to the model and required precision.

Final check: never mix kilograms and newtons.

Beginner checkpoint — which quantity is the number describing?

If the value is in kilograms, it is a mass. If it is in newtons, it is a force and may be a weight. If it is in newtons per kilogram, it describes gravitational field strength. Keeping the unit attached to every number prevents the common statement that an object “becomes 38% of its kilograms” on Mars.

For a 100 kg cargo item, the mass remains 100 kg. Near the Martian surface, its weight is about 371 N using g = 3.71 N/kg. The lower weight helps with lifting and support loads, but the 100 kg mass still determines inertia. That distinction matters for vehicle braking, robotic arms, cranes and collision safety.

8 — Corrected exercises

Exercise 1

25 kg battery on Mars?

25 × 3.71 = 92.75 N.

Exercise 2

Does 100 kg become 38 kg on Mars?

No. Mass remains 100 kg; weight is about 371 N.

AM-02.03 — Mass, weight, and gravity: why 80 kg does not become 13 kg on the Moon