The weightThe force acting on an object due to the pull of gravity from a massive object like a planet. The force acts towards the centre of the planet and is measured in newtons (N). of an object may be thought of as acting at a single point called its centre of massThe point representing the mean position of the matter in a body.. Depending on the object's shape, its centre of mass can be inside or outside it.
The weight of an object and its mass are directly proportional. For a given gravitational field strength, the greater the mass of the object, the greater its weight.
weight = mass × gravitational field strength
\(W = m~g\)
This is when:
weight (W) is measured in newtons (N)
mass (m) is measured in kilograms (kg)
gravitational field strength (g) is measured in newtons per kilogram (N/kg)
Example
An apple has a mass of 100 g. Calculate its weight on Earth (g = 9.8 N/kg).
\(100~g = \frac{100}{1000} = 0.1~kg\)
\(W = m~g\)
\(W = 0.1~kg \times 9.8~N/kg\)
\(W = 0.98~N\)
Question
Calculate the weight of a 30 kg dog (g = 9.8 N/kg).