Weight (OCR A Level Physics)

Weight

• Weight is the effect of a gravitational field on a mass

• Since it is a force on an object due to the pull of gravity, it is measured in Newtons (N) and is a vector quantity

• The weight of a body is equal to the product of its mass (m) and the acceleration of free fall (g)

• g is the acceleration due to gravity or the gravitational field strength

• On Earth, this is 9.81 m s⁻² (or N kg⁻¹)

Free fall

• An object in free fall is falling solely under the influence of gravity

• On Earth, all free-falling objects accelerate towards Earth at a rate of 9.81 m s⁻²

• In the absence of air resistance, all bodies near the Earth fall with the same acceleration regardless of their mass

Mass v Weight

•  An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field on different planets

• For example, the gravitational field strength on the Moon is 1.63 N kg^-1, meaning an object’s weight will be about 6 times less than on Earth

On the moon, a person's mass will stay the same but their weight will be much lower

• The value of g on other planets in the solar system is given in the diagram below

  • Notice how much this varies according to the size of the planet

Gravitational field strength of the planets in our solar system