Mass and weight are different — Physics, 11–13
Mass tells how much matter an object has; weight is the pull of gravity on it. Physics, 11–13 years.
Idea
Mass is how much matter an object contains, so it stays the same wherever the object goes. Weight is a force caused by gravity pulling on that mass. A backpack has the same mass on Earth and the Moon, but it weighs less on the Moon because the Moon’s gravity is weaker.
Why
People needed two ideas because an object can travel to a place with different gravity without gaining or losing any material. A balance compares masses, while a spring scale measures the pulling force we call weight. Separating them explains why astronauts can lift equipment more easily on the Moon.
Worked example
A 5 kg bag is on Earth, where gravity is about 10 N for each kilogram. Step 1: identify the mass: 5 kg. Step 2: calculate weight: 5 × 10 = 50 N. If the same bag went to the Moon, its mass would still be 5 kg, but its weight would be about 8 N because lunar gravity is about one sixth of Earth’s.
Common trap
It is easy to say that a person’s weight is 50 kg, because bathroom scales show a number in kilograms. In science, kilograms measure mass, while weight is a force measured in newtons. The everyday wording is understandable: the scale senses the force of gravity, then converts it into an estimated mass for us.
In the world
Engineers use mass when calculating how much material a bridge or spacecraft contains, and weight when checking the forces its supports must hold. Lifting equipment is also designed for the local gravity. A crane on Earth must support more weight than the same crane would need to support on the Moon.
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