Weight (Edexcel IGCSE Physics)
Revision Note
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Weight
Weight is defined as:
The force experienced by an object with mass when placed in a gravitational field
Weight and mass
Weight and mass are different in physics
Mass is a measure of how much matter there is in an object
Mass has magnitude but not direction
Therefore, mass is a scalar quantity
Weight is a force
Forces have magnitude and direction
Therefore, weight is a vector quantity
Gravitational field strength
Planets have strong gravitational fields
Hence, they attract nearby masses with a strong gravitational force
Different planets have different gravitational field strengths
This depends on the mass of the planet
More massive planets have stronger gravitational field
Weight, mass and gravitational field strength
Because of weight:
Objects stay firmly on the ground
Objects will always fall to the ground
Satellites are kept in orbit
Some of the phenomena associated with gravitational attraction and the weight force
Weight, mass and gravitational field strength are related using the equation:
Where:
= weight, measured in newtons (N)
= mass, measured in kilograms (kg)
= gravitational field strength, measured in newtons per kilogram (N/kg)
The gravitational field strength on Earth is 10 N/kg
is also used to describe the acceleration of an object in freefall in a gravitational field
The acceleration of freefall on Earth is 10 m/s2
These quantities are two ways of describing the thing
The weight that an object experiences depends on:
The object's mass
The mass of the planet attracting it
The mass of an object and the weight acting on it are directly proportional
If one doubles, the other also doubles
If one is halved, the other is also halved
The magnitude of the weight force depends on the gravitational field strength
Worked Example
NASA's Artemis mission aims to send the first woman astronaut to the Moon. Isabelle hopes to one day become an astronaut. She has a mass of 40 kg.
Earth's gravitational field strength is 10 N/kg, and the Moon's gravitational field strength is 2 N/kg.
Discuss the difference between Isabelle's weight on Earth, and her weight on the Moon.
Answer:
Step 1: State the equation linking weight and mass
The equation linking weight and mass is:
W = m × g
Step 2: List the known values
Earth's gravitational field strength, g = 10 N/kg
The Moon's gravitational field strength, g = 2 N/kg
Mass, m = 40 kg
Step 3: Calculate Isabelle's weight on Earth
Substituting the values of mass and Earth's gravitational field strength into the equation gives:
W = 40 × 10 = 400 N
Step 4: Calculate Isabelle's weight on the Moon
Substituting the values of mass and the Moon's gravitational field strength into the equation gives:
W = 40 × 2 = 80 N
Step 5: Discuss the two values of weight
Isabelle's weight is greater on Earth than on the Moon
This is because the Earth has a larger gravitational field strength than the Moon, so Isabelle's weight force (the force of gravity pulling down on her) is larger on Earth than on the Moon
Examiner Tips and Tricks
It is a common misconception that mass and weight are the same, but they are in fact very different
Since weight is a force - it is a vector quantity
Since mass is an amount - it is a scalar quantity
You do not need to remember the value of g on Earth; it will be given to you in the exam.
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