Calculating Weight (AQA GCSE Physics): Revision Note
Exam code: 8463
Mass v Weight
Mass (measured in kilograms, kg) is related to the amount of matter in an object
Weight (measured in newtons, N) is the gravitational force exerted on an object with mass
The weight of an object and the mass of an object are directly proportional
The size of the weight force depends on the gravitational field strength (often called g for short)
Examiner Tips and Tricks
It is a common misconception that mass and weight are the same, but they are in fact very different
Weight is a force, with both magnitude and direction, therefore it is a vector quantity
Mass has magnitude but no direction, therefore it is a scalar quantity
Calculating Weight
Weight, mass and gravitational field strength are related using the equation:
g is known as the acceleration due to gravity or the gravitational field strength
On Earth, this is equal to 9.81 m/s2 (or N/kg)
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/s2
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, meaning an object’s weight will be about 6 times less than on Earth
On the moon, your mass will stay the same but your weight will be much lower
The value of g (gravitational field strength) varies from planet to planet depending on their mass and radius
A few examples of varying gravitational field strength are shown below:
Gravitational field strength of the planets in our solar system
Worked Example
A student estimates they would have a weight of 190 N on Mars. The gravitational field strength on Earth is 9.8 N/kg. The gravitational field strength on Mars is 3.8 N/kg. Calculate the weight of the student on Earth.
Answer:
Step 1: List the known quantities
Weight on Mars, WM = 190 N
Gravitational field strength on Mars, gM = 3.8 N/kg
Gravitational field strength on Earth, gE = 9.8 N/kg
Step 2: Write out the equation relating mass and weight and rearrange for mass
W = mg
Divide both sides by g:
Step 3: Calculate the student’s mass
The student’s mass is the same anywhere in the universe
Step 4: Calculate the student’s weight on Earth
WE = m × gE = 50 × 9.8 = 490 N
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