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
Worked example
The acceleration due to gravity on the moon is 1/6 of that on Earth. If the weight of a space probe on the moon is 491 N, calculate its mass.
Examiner Tip
- It is a common misconception that mass and weight are the same, but they are in fact very different
- Weight is the force of gravity acting upon an object
- Weight is a vector quantity
- Mass is the amount of matter contained in the object
- Mass is a scalar quantity
