Weight and the effects of forces
The weight of an object
A specific force: weight
The weight of an object is the force of attraction exerted by the Earth on that object. It is a force at a distance: there is no need to touch the object for it to act, it is present permanently, from ground level up to the top of a mountain (it varies very little).
Weight is denoted P (vector P) and is measured in newtons (N), with a dynamometer.
Weight and mass: two different concepts
Weight and mass are often confused, but they are two quite distinct quantities.
| Quantity | Symbol | Unit | Measured with | Varies by location? |
|---|---|---|---|---|
| Mass | m | kilogram (kg) | Scale | No, it is constant |
| Weight | P | newton (N) | Dynamometer | Yes, it depends on gravity |
An astronaut has the same mass on the Moon and on Earth, but their weight is about 6 times smaller on the Moon, because lunar attraction is weaker.
The relationship between weight and mass
On Earth, weight and mass are related by the formula:
P = m x g
where:
- P is the weight in newtons (N),
- m is the mass in kilograms (kg),
- g is a constant called the gravitational field strength, which is about 10 N/kg on Earth.
Example calculation
A 5 kg bag therefore has a weight of: P = 5 x 10 = 50 N
Classic pitfall to avoid
Never say "I weigh 40 kilos" while thinking of a force: the kilogram measures mass, not weight! In everyday language the two are mixed up, but in physics you must clearly distinguish mass (in kg, with a scale) and weight (in N, with a dynamometer).