Weight
Brook Edgar & Hannah Shuter
Teachers
Contents
Explainer Video
Mass vs Weight
Understanding Mass:
Mass is the amount of matter or "stuff" that an object contains. Think of it as how much material makes up an object. An elephant contains more matter than a mouse, so it has a greater mass.
Mass is measured in kilograms , though you might also see it in other units, such as grams .
Mass never changes. Whether you're on Earth, the Moon, Mars, or floating in space, your mass stays exactly the same. You're made of the same amount of matter wherever you go
Understanding Weight:
Weight is distinct from mass. Although people often use the words interchangeably in everyday life - in physics, this is not allowed.
Weight is the force acting on an object due to gravity. It's measured in newtons (), not kilograms
Weight is a force, and like all forces, it is a vector (it has both magnitude and direction)
Gravity is an attractive force, so always acts towards the centre of the planet
Here's the key distinction: unlike mass, weight can change depending on where you are. Your weight on the Moon would be much less than on Earth because the Moon has weaker gravity, but your mass would be exactly the same.
insert Earth and Moon image
The Weight Equation
Weight can be calculated using this equation.
Formula:
Gravitational field strength tells you how strong gravity is at a particular location. On Earth, . You need to remember this number for your GCSEs. This means that every kilogram of mass experiences a downward force of .
Different planets and moons have different gravitational field strengths. For example: on Mars , and on the Moon .
Example: My cat Erwin has a mass of . If I want to calculate his weight (or the force on him due to gravity), I could use the above equation:
Note that we aren't told in the question that the gravitational field strength on Earth is - we need to recall it.
If Erwin went on holiday to Mars, his mass would remain the same but his weight would be less as the gravitational field strength of Mars is less than that on Earth.
Remember: Unit conversions! The standard unit of mass is - so if mass is in you need to convert it.
Worked Example:
A child has a mass of . On Earth . What is the weight of the child?
Answer:
Worked Example:
The value of on Earth is , find the following:
The weight of a puppy with a mass of .
The mass of a train with weight .
The mass of a cat with weight .
Answer:
Teacher tips: Remember to convert units and substitute numbers in before rearranging.
Worked Example:
On Mars, . What is the weight of an man on Mars.
Answer:
Gravity is a Non-Contact Force
Gravity is an attractive non-contact force that acts between all objects with mass. It is:
Attractive - meaning it always pulls objects together, never pushes them apart
Non-contact - meaning objects don't need to be touching for the force to act
The force that pulls you down towards the centre of the planet and keeps you on the surface of the Earth.
image here
Worked Example:
Choose the correct answer:
Which statement is true?
A contact force is when objects are not touching.
A non-contact force is when objects are not touching.
Friction is a non-contact force.
Answer:
is wrong because a contact force is when objects are touching.
is correct because a non-contact force is when objects are not touching.
is wrong because friction is a contact force.
Worked Example:
Choose the correct answer:
Which statement is true?
Gravity acts between objects with mass.
Gravity is a contact force.
Gravity is measured in n.
Answer:
is correct, gravity acts between objects with mass.
Gravity is incorrect because gravity is a non-contact force -> If you jump in the air, gravity still acts to bring you back down to earth.
is incorrect because gravity is measured in newtons, not 'n'. 'n' is the prefix for 'nano' in measurements, meaning .
Practice Questions
An astronaut on Earth has a mass of . The gravitational field strength on Earth is .
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State the equation that links weight, mass and gravitational field strength.
Calculate the weight of the astronaut on Earth.
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The astronaut travels to the Moon, where the gravitational field strength is . Explain what happens to their mass and their weight.
→ Check out Hannah's video explanation for more help.
Answer:
Mass stays the same at . Weight decreases because is smaller, so is smaller on the Moon.
A student uses a newton-meter to measure the weight of an object.
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The object has a mass of Calculate its weight on Earth.
Explain why weight is considered to act at the object’s centre of mass.
-> Check out Hannah's video explanation for more help.
Answer:
The centre of mass is the point where the object’s mass can be thought of as concentrated. Weight acts effectively at this single point, even though mass is distributed through the object.
Common Errors and Exam Tips
Remember to convert grams → kilograms
Know that mass doesn’t change (unless you’re eating more or going to the gym!), but weight does depending on location
Remember that on Earth is