Weight, Mass and Gravity

• Gravitational Force


  The Force of Attraction Between Masses
• Gravity attracts all masses, but you only notice it when one of the masses is really really big, e.g. a planet
• Effects of gravity near a planet or star

  • It makes all things fall towards the ground
  • It gives everything a weight
• Mass


  The amount of 'stuff' in an object, which has the same value anywhere in the universe
• Weight


  The force acting on an object due to gravity (the pull of the gravitational force on the object)
• Gravitational field strength


  • 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
• An object has the same mass whether it's on Earth or on the Moon, but its weight will be different
• Centre of mass


  A point at which you assume the whole mass is concentrated, for a uniform object this will be at the centre of the object
• Weight measurement


  Measured using a calibrated spring balance (or newtonmeter)
• Mass measurement


  Measured in kilograms with a mass balance (an old-fashioned pair of balancing scales)
• Calculating weight


  Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
• For Earth, g = 9.8 N/kg and for the Moon it's around 1.6 N/kg
• Increasing the mass of an object


  Increases its weight
• Direct proportionality


  Weight and mass are directly proportional, written as W ∝ m