Weight, mass and gravity
Weight
is the acting on an object because of . Close to the Earth, gravity comes from the Earth's gravitational field. You must recall this equation:
, or W = mg
| Quantity | Symbol | Unit |
|---|---|---|
| Weight | W | newtons (N) |
| Mass | m | kilograms (kg) |
| Gravitational field strength | g | newtons per kilogram (N/kg) |
On Earth, g = 9.8 N/kg. On the Moon it is about 1.6 N/kg.
Example: find the weight of a 60 kg student on Earth.
- W = mg
- W = 60 × 9.8
- W = 588 N
On the Moon, the same student weighs 60 × 1.6 = 96 N.
Key terms
Mass and weight
is the amount of matter in an object, in kilograms. It is the same wherever the object is.
is a force, in . It depends on the gravitational field strength where the object is.
Weight and mass are (W ∝ m): double the mass and the weight doubles.
You measure weight with a calibrated spring balance, called a .
Common mistake
Mass and weight are not the same thing. "I weigh 60 kg" is wrong in physics: your mass is 60 kg and your weight is about 588 N.
Key terms
Centre of mass
The of an object is the single point where its can be thought of as acting.
For a regular, even object, such as a ruler or a ball, the centre of mass is at its centre. A ruler balances on your finger when your finger is right under its centre of mass.
When you draw the weight of an object as an arrow, start it at the centre of mass and point it straight down.