Beginner · 8 min
Mass, weight, and gravity
Distinguish mass from weight and calculate the force of gravity.
Mass and weight are different
Mass measures an object's inertia: how strongly it resists a change in velocity. It is measured in kilograms (kg). Weight is the gravitational force acting on that mass and is measured in newtons (N). Moving an object from Earth to the Moon changes its weight because gravity is weaker there. Its mass stays the same.
Worked example
A bag has a mass of 4 kg on Earth and on the Moon. Its weight is smaller on the Moon.
Calculate weight
Weight equals mass multiplied by gravitational field strength: W = m × g. Near Earth's surface, g is about 9.8 N/kg, equivalent to a free-fall acceleration of 9.8 m/s² when air resistance is ignored. We use a rounded value of 10 N/kg in these exercises to make the arithmetic simpler. Always use the value stated in the question.
Worked example
For a 6 kg object with g = 10 N/kg: Weight = 6 × 10 = 60 N
Compare two worlds
- Calculate the weight of a 5 kg object where g = 10 N/kg.
- Calculate its weight where g = 2 N/kg. State which quantity stays unchanged: mass or weight.