Mass and Weight

Mass is a measure of the amount of matter in an object. It is a scalar quantity, measured in kilograms (kg). A 2 kg bag of sugar has 2 kg of mass on Earth, on the Moon, on Mars, or floating in deep space.

Weight is the force of gravity acting on an object's mass. It is a vector quantity (force has direction — pointing toward the centre of the planet), measured in newtons (N). The same 2 kg bag of sugar would weigh:

• about 19.6 N on Earth (g ≈ 9.8 N/kg)
• about 3.2 N on the Moon (g ≈ 1.6 N/kg)
• 0 N in deep space far from any planet or star.

The link is:

$W = m \, g$

where W is weight (N), m is mass (kg) and g is the gravitational field strength (N/kg) at that location.

Location	g (N/kg)
Earth's surface	9.8
Moon's surface	1.6
Mars's surface	3.7
Jupiter (cloud top)	24.8

A top-pan balance compares the unknown mass to a known reference, so it gives the same reading anywhere — on Earth or on the Moon. It measures mass (kg).

A newton-meter (spring balance) hangs the object on a spring and measures how much the spring stretches. Because the stretch depends on the gravitational pull, the reading changes if you move it. It measures weight (N).

Instrument	Measures	Earth → Moon
Top-pan balance	mass (kg)	same reading
Newton-meter	weight (N)	reads ~⅙ as much

Bathroom scales actually measure weight (in newtons internally), then divide by 9.8 to display a mass in kg — assuming you are on Earth. The display would be very wrong on the Moon.

Inertia is the tendency of an object to keep doing what it is doing — staying still if at rest, or moving in a straight line at a steady speed if moving.

The larger the mass, the larger the inertia, and the harder it is to start, stop or turn the object. Pushing a shopping trolley with a 2 L bottle in it is easy; pushing the same trolley with a 20 kg bag of cement is much harder, even though gravity has the same direction.

In space (where weight is effectively zero), inertia is still there. Astronauts inside the ISS still have to push and pull to make heavy objects move and stop. Their mass has not changed — only their weight.