📖 In-Depth Theory
Weight and Gravitational Field Strength
WEIGHT is the FORCE acting on an object due to GRAVITY — it is a vector quantity acting downward.
WEIGHT EQUATION:
W = m × g
W = weight (newtons, N)
m = mass (kilograms, kg)
g = gravitational field strength (N/kg)
On Earth: g = 9.8 N/kg (use 10 N/kg for estimates).
On the Moon: g ≈ 1.6 N/kg (about 1/6 of Earth's).
MEANING OF g:
The gravitational field strength g tells us the weight per unit mass.
On Earth: every 1 kg of mass experiences 9.8 N of gravitational force downward.
Mass vs Weight
MASS:
Amount of matter in an object.
Scalar quantity — measured in kilograms (kg).
Constant everywhere in the universe — doesn't change with location.
WEIGHT:
Gravitational force on an object.
Vector quantity — measured in newtons (N).
Changes with location — depends on gravitational field strength.
EXAMPLES:
A 70 kg astronaut:
On Earth: W = 70 × 9.8 = 686 N
On the Moon: W = 70 × 1.6 = 112 N
In deep space (no gravity): W = 0 N
Mass is 70 kg everywhere.
MEASURING:
Mass: measured with a balance (compares gravitational force on both sides — same anywhere).
Weight: measured with a calibrated spring balance/newton meter (reads force directly).
Gravitational Fields
A GRAVITATIONAL FIELD is a region around a mass where any other mass experiences a force.
Field lines point TOWARDS the centre of mass (gravity is always attractive).
On Earth's surface, field lines point vertically downward (towards Earth's centre).
Gravitational field strength DECREASES with distance from the mass:
At Earth's surface: g = 9.8 N/kg
At altitude of 400 km (ISS orbit): g ≈ 8.7 N/kg (astronauts are still falling — not weightless!)
WHY 'WEIGHTLESSNESS' IN SPACE:
Astronauts in orbit are NOT weightless in terms of gravitational force.
They are in FREE FALL — constantly falling towards Earth but moving sideways fast enough to miss it.
The sensation of weightlessness occurs because everything around them is falling at the same rate.
⚠️ Common Mistake
Mass and weight are DIFFERENT things. Mass (kg) is constant everywhere. Weight (N) changes with gravitational field strength. On the Moon, your MASS stays the same but your WEIGHT is less. Weight is measured in NEWTONS — not kilograms.