Forces: Weight, Mass and Gravity

Weight, Mass, and Gravity

Mass

• The mass of an object is a measure of the amount of matter in the object. It is a scalar quantity and remains unchanged regardless of where the object is in the universe.

• Mass is measured in kilograms (kg).

Weight

• Weight is a force acting on an object due to gravity. It is a vector quantity, meaning it has both magnitude (size) and direction - it always acts towards the centre of the planet.

• Since weight is a force, it is measured in newtons (N).

• An object’s weight can change depending on where it is in the universe - it would be less on the Moon than on Earth, for example, due to the lower gravitational field strength.

• Weight can be calculated using the formula: Weight (N) = mass (kg) x gravitational field strength (N/kg).

Gravity

• Gravity is a force that attracts two bodies towards each other. The gravitational pull between an object and the Earth causes the object to have weight.

• The gravitational field strength on Earth is approximately 9.8 N/kg - this means that an object with a mass of 1 kg would have a weight of 9.8 N on Earth.

• The gravitational field strength is different on other planets and celestial bodies; it is less on the Moon and Mars for example, so an object would weigh less on these bodies.

• The force of gravity decreases the further away an object is from the planet’s centre. Therefore, an object would weigh slightly less at the top of Mount Everest than it would at sea level because it is slightly further away from the centre of the Earth.