Weight, Mass & Gravity (AQA GCSE Physics: Combined Science)

• Weight is defined as:

The force acting on an object due to gravitational attraction

• Planets have strong gravitational fields
  • Hence, they attract nearby masses with a strong gravitational force

• Because of weight:
  • Objects stay firmly on the ground
  • Objects will always fall to the ground
  • Satellites are kept in orbit

Some of the phenomena associated with gravitational attraction and the weight force

• Mass (measured in kilograms, kg) is related to the amount of matter in an object
  • The more mass an object has the larger the weight force it will experience

• Since weight is a force, it is measured in Newtons (N)
• The weight that an object experiences depends on:
  • The object's mass
  • The mass of the planet attracting it

• Mass is commonly measured using a top pan balance
  • The weight can then be indirectly found through calculation

• Weight can be measured directly using a calibrated spring-balance, also known as a newton-meter
  • This device is a type of weighing scale which measures force in Newtons
  • It consists of a spring fixed at one end with a hook to attach an object at the other

Weight can be measured indirectly using a top-pan balance or directly using a newton-meter

• The centre of mass of an object (sometimes called the centre of gravity) is defined as:

The point through which the weight of an object acts

• For a symmetrical object of uniform density, the centre of mass is located at the point of symmetry
  • For example, the centre of mass of a sphere is at the centre

The centre of mass of a regular shape can be found by symmetry

• The centre of mass of an irregular object can be found by locating its balance point
  • A broomstick has a centre of mass slightly closer to the head of the broom since there is more mass located there

The centre of mass of a broomstick which is also its balance point