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Mass is the amount of matter in an object, measured in kg, while weight is the force of gravity on it, measured in newtons. Free GCSE Physics notes.
Introduction to Gravity and Weight is part of Forces and their interactions in GCSE AQA Higher Physics.
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Mass is the amount of matter in an object, measured in kg, while weight is the force of gravity on it, measured in newtons. Free GCSE Physics notes.
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Use this lesson to stay inside Forces and their interactions while connecting the detail back to the wider Forces topic.
Read the core explanations from Introduction to Gravity and Weight before testing yourself from memory.
You step on bathroom scales and they show '70 kg'. But here's the thing - those scales aren't actually measuring your mass at all. They're measuring your weight, then converting it to show mass. This confusion between weight and mass trips up countless students in exams.
Mass is the amount of stuff you're made of - your atoms, molecules, everything. It stays exactly the same whether you're on Earth, the Moon, or floating in space. Weight, however, is the pull of gravity on that mass. Change the gravity, and your weight changes dramatically.
Think of it this way: if you travelled to the Moon tomorrow, you'd still be the same person with the same mass. But you'd weigh only one-sixth as much because the Moon's gravity is weaker. Your bathroom scales would show a much smaller number, but you haven't lost any actual matter.
Earth doesn't just sit there doing nothing - it creates an invisible force field around itself called a gravitational field. Any object with mass placed in this field experiences a force pulling it towards Earth's centre. The stronger the field, the greater the pull.
We measure the strength of this field using gravitational field strength, given the symbol g. On Earth's surface, g = 9.8 N/kg. This means every kilogram of mass experiences a 9.8 newton force pulling it downwards. Double the mass, double the force.
But here's where it gets interesting - gravitational field strength isn't the same everywhere. It's stronger at sea level than on top of Mount Everest. It's different on other planets entirely. Jupiter's massive size creates g = 25 N/kg, while tiny Mercury only manages g = 3.7 N/kg.
The beauty of gravitational field strength is that it tells us exactly how much force acts on any mass in that location. Know the field strength and the mass? You can calculate the weight instantly using our key equation.
Now we can connect everything together with the fundamental weight equation. Weight equals mass multiplied by gravitational field strength. In symbols: W = mg. This simple equation is your key to solving every weight problem in GCSE physics.
Notice the units work perfectly. Mass in kg multiplied by gravitational field strength in N/kg gives weight in N. The kg units cancel out, leaving just newtons - exactly what we want for a force.
This equation reveals why weight and mass are directly proportional. Double the mass, and you double the weight (assuming g stays constant). This proportional relationship is crucial for understanding how objects behave under gravity.
Worked example: Calculating Weight on Earth
Calculate the weight of a 50 kg student on Earth where g = 9.8 N/kg. [2 marks]
Answer: 490 N
Every object has weight, but where exactly does this weight act? You might think it's spread out evenly, but physics has a more elegant solution. We can treat all the weight as acting at a single point called the centre of mass.
For regular, uniform objects like spheres or cubes, the centre of mass sits right in the geometric centre. But for irregular shapes or objects with uneven density, the centre of mass might be off to one side, or even outside the object entirely.
This concept is incredibly useful for calculations. Instead of worrying about the weight of every tiny part of an object, we can treat the entire weight as a single force acting downwards from the centre of mass. It simplifies complex problems dramatically.
Think about balancing a ruler on your finger. The balance point is the centre of mass. If you hang weights from different positions on the ruler, the centre of mass shifts towards the heavier side. This is why a loaded truck has a different centre of mass than an empty one.
How do we actually measure weight in practice? The answer is the spring balance, also called a newtonmeter. This device uses Hooke's law - the idea that a spring stretches proportionally to the force applied to it.
When you hang an object from a spring balance, the object's weight pulls the spring downwards. The spring stretches until its upward elastic force exactly balances the downward weight. The amount of stretch indicates the size of the weight force.
The key word here is 'calibrated'. The spring balance must be properly calibrated to give accurate readings. This means the scale has been marked using known weights, so when the spring stretches by a certain amount, we know exactly what force caused that stretch.
Spring balances measure weight directly in newtons, unlike bathroom scales which measure weight but display the result in kilograms. This makes spring balances perfect for physics experiments where we need to know the actual force, not just the equivalent mass.
Weight questions are mark-winners if you know the tricks, but they're also full of traps. The biggest mistake students make is confusing weight and mass. Remember: weight is a force measured in newtons, mass is an amount of matter measured in kilograms.
When calculating weight, always show your working clearly. Write W = mg, substitute the numbers with units, then calculate. Examiners give method marks for the correct approach even if your final answer is wrong. Miss out the formula and you lose marks immediately.
Watch out for unit conversions. If mass is given in grams, convert to kilograms first. If gravitational field strength is given as 10 N/kg instead of 9.8 N/kg, use the value they give you - don't assume it's always 9.8.
For questions about centre of mass, focus on the key idea that weight acts at a single point. Don't overcomplicate it - you're not expected to calculate exactly where the centre of mass is, just understand that it exists and why it's useful.
Worked example: Comparing Weight on Different Planets
A 60 kg astronaut has a weight of 588 N on Earth (g = 9.8 N/kg). Calculate their weight on Mars where g = 3.7 N/kg. [3 marks]
Answer: 222 N (much less than on Earth due to weaker gravity)
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Mass is the amount of matter in an object, measured in kilograms (kg), and it is the same everywhere. Weight is the gravitational force acting on an object, measured in newtons (N), and it varies with gravitational field strength, so an object weighs less on the Moon.
Gravitational field strength (g) is the gravitational force per unit mass acting on objects in a gravitational field, measured in N/kg. On Earth's surface g = 9.8 N/kg, so every kilogram experiences a 9.8 newton force; on Jupiter g = 25 N/kg.
The centre of mass is the point where the total weight of an object can be considered to act. For regular, uniform objects it is at the geometric centre, but for irregular shapes it may be off to one side or even outside the object.
Weight is measured with a calibrated spring balance, also called a newtonmeter. The object's weight stretches the spring until the spring's upward elastic force balances it, and the amount of stretch shows the weight in newtons.
A gravitational field is the invisible force field around Earth in which any object with mass experiences a force pulling it towards Earth's centre. The stronger the field, the greater the pull, measured by gravitational field strength, g.
The gravitational field strength on Jupiter is 25 N/kg, because of its massive size, compared with 9.8 N/kg on Earth's surface. Mercury has only 3.7 N/kg.
People weigh less on the Moon because the Moon's gravity is weaker, so they weigh only one-sixth as much as on Earth. Their mass stays exactly the same, because mass is the amount of matter, while weight is the pull of gravity on that mass.
Gravitational field strength is not the same everywhere: it is stronger at sea level than on top of Mount Everest, and different on other planets entirely. On Earth's surface g = 9.8 N/kg.
A newtonmeter, or spring balance, measures weight directly in newtons. The object's weight stretches a spring until the spring's upward elastic force balances it, and the amount of stretch shows the size of the weight.
A spring balance needs to be calibrated so it gives accurate readings: its scale is marked using known weights, so when the spring stretches by a certain amount, the force that caused it is known. It works using Hooke's law, where the stretch is proportional to the force.
Bathroom scales measure weight and then convert it to show mass in kilograms. A spring balance measures weight directly in newtons, which makes it better for physics experiments.
A 60 kg astronaut on Mars, where g = 3.7 N/kg, has a weight of 222 N, much less than their 588 N weight on Earth. Mass stays the same everywhere, and W = mg = 60 × 3.7 = 222 N.
For irregular shapes or objects with uneven density, the centre of mass might be off to one side, or even outside the object entirely. For regular, uniform objects like spheres or cubes, it sits right in the geometric centre.
Weight is directly proportional to mass when gravitational field strength stays constant, because W = mg. Double the mass and the weight doubles; on Earth every kilogram experiences 9.8 N.