Weight, Mass and Gravity

• Gravity attracts all masses but you only notice it when one of the masses is really big. Anything near a planet or a star is attracted to it very strongly.

  This has two important effects:

  1. On the surface of a planet, it makes all things fall to the ground
  2. It gives everything a weight
• Mass is the amount of ‘stuff’ in an object. For any given object this will have the same value anywhere in the universe
• Weight is the force acting on an object due to gravity. Close to Earth, this force is caused by the gravitational field around Earth.

  Gravitational field strength varies with location. It’s stronger the closer you are to the mass causing the field and stronger for larger masses
• The weight of an object depends on the strength of the gravitational field at the location of the object. This means that the weight of an object changes with its location
• Weight is a force measured in Newtons. You can think of the force as acting from a single point on the object called its centre of mass (whole mass is concentrated). For a uniform object, this will be at the centre of the object.

  Weight is measured using a calibrated spring balance (or newtonmeter)
• Mass is not a force. It’s measured in kilograms with a mass balance
• Weight (N) = Mass (kg) x Gravitational field strength (N/kg)
• Increasing the mass of an object increases its weight. If you double the mass, the weight doubles too so you can say that weight and mass are directly proportional