Forces and Energy Changes: Mass and Weight

Mass refers to the amount of matter in a body, which is constant everywhere in the universe. It is measured in kilogrammes (kg).
Weight is a force that depends on both the mass of an object and the strength of the gravitational field it is in. Weight thus changes depending on where in the universe the object is.
Weight is calculated by the equation Weight = mass x gravitational field strength (W=mg). Mass is measured in kilogrammes, gravitational field strength in newtons per kilogramme (N/kg), and weight is measured in newtons (N).
The gravitational field strength on Earth is approximately 9.8 N/kg close to the Earth’s surface.
When weight is acting downwards, it is often the force that causes an object to have potential energy. This energy can be converted into kinetic energy when the object is dropped or falls.
Friction and air resistance can reduce the amount of kinetic energy that an object has, turning some of it into heat energy.
Weight, being a force, is a vector quantity. This means it not only has magnitude (size) but also direction - it acts towards the centre of the mass causing the gravity.
Work done (energy transferred) by forces can be calculated using the equation Work done = force x distance (W = Fd). This applies when the force is acting in the same direction as the motion.
The stronger the gravity, the more work needs to be done to lift an object and hence, the object will have more potential energy.
The ‘mass x height’ or ‘mgh’ term you see in gravitational potential energy equations is directly related to the work done in lifting the object to that height against the force of gravity.