# Forces and Motion: Mass, Weight and Gravity

## Forces and Motion: Mass, Weight and Gravity

• The term ‘mass’ refers to the quantity of matter in a substance or object. It’s expressed in kilogrammes (Kg) and does not change, regardless of where the object is within the universe.

• ‘Weight’, on the other hand, is the force of gravity acting on a mass. Since weight depends on both mass and gravity, it will vary depending on where in the universe the mass is located.

• Weight is measured in newtons (N). It can be calculated using the following formula: Weight (N) = Mass (Kg) * Gravitational Field Strength.

• Gravitational field strength is approximately 10 N/kg on Earth. This means that every kilogramme of mass experiences a gravitational force of about 10 N on earth.

• However, gravitational field strength can vary depending on location. For example, it’s approximately 1.6 N/kg on the Moon. Thus, an object would weigh less on the Moon compared to the Earth.

• An object experiences weightlessness when it is in freefall. That’s because although the object is under the influence of gravity, there are no forces opposing this gravity, creating the sensation of weightlessness.

• Skeleton diagrams are used to display forces acting on an object. In these diagrams, the weight acts downwards from the centre of gravity.

• Gravity is an attractive force that affects all matter, pulling objects towards each other. The bigger the masses of the objects, the bigger the force. The further apart the masses, the weaker the force.

• The effects of gravity can be observed in freefall acceleration, orbital motion of celestial bodies and in the focus of mass of an object or a system of objects.