Forces are everywhere around you - from walking down the... Show more
Physics102 views·Updated May 29, 2026·5 pagesGCSE Physics Edexcel Topic 2: Motion Notes (Combined and Triple)
Siddharth Sriram@sidlovestransprt
1 / 5
1
of 5
Forces and Newton's First Law
Every time you push, pull, or bump into something, you're applying a force, measured in Newtons (N). When multiple forces act on an object, they combine to create a resultant force - think of it as the 'winning' force after all forces battle it out.
Here's the key bit: if forces are balanced , nothing changes. If they're unbalanced, things start moving or change speed. A book sitting on your desk has balanced forces - gravity pulls down whilst the desk pushes up with equal force.
Newton's First Law tells us that objects are lazy - they don't want to change what they're doing. This laziness is called inertia, and it's why you lurch forward when a bus suddenly stops.
Quick Check: A football rolling on grass gradually slows down because friction (an external force) acts against its motion, proving Newton's First Law!
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of 5
Mass, Weight, and Newton's Second Law
Don't confuse mass and weight - your mass (measured in kg) stays the same whether you're on Earth or the Moon, but your weight (measured in N) changes because weight is what happens when gravity acts on your mass.
Newton's Second Law gives us the famous equation: F = ma . This means heavier objects need more force to speed up, and lighter objects accelerate more easily with the same force.
The mathematical relationship shows that force is proportional to the rate of change in momentum. Essentially, if you want to change how fast something's moving, you need to apply force - and the bigger the change you want, the more force you'll need.
Real-world example: It takes much more force to accelerate a lorry than a bicycle because the lorry has significantly more mass!
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of 5
Newton's Third Law
Newton's Third Law is probably the most misunderstood: "For every action, there is an equal and opposite reaction." This doesn't mean forces cancel out - they act on different objects entirely.
When you sit on a chair, you push down with 200N whilst the chair pushes up on you with 200N. These forces are equal in size but opposite in direction. You don't fly off because the forces balance, but if you applied more force than the chair could handle, it would break.
This law explains how rockets work in space (they push gas backwards, so the gas pushes the rocket forwards) and why you can walk (you push backwards on the ground, so the ground pushes you forwards).
Think about it: When you jump, you push down on the Earth and it pushes back up on you - but you move more because you have much less mass than the planet!
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Momentum and Stopping Distances
Momentum measures how hard it is to stop a moving object. A massive lorry moving slowly can have the same momentum as a small car moving quickly, making both equally difficult to stop.
The conservation of momentum means the total momentum before a collision equals the total momentum after. When two cars crash and stick together, they share their momentum between them.
Stopping distance for cars has two parts: thinking distance (how far you travel whilst reacting) and braking distance (how far you travel whilst the brakes work). Human reaction time is typically 0.25 seconds, but distractions like phones or alcohol make this much worse.
Safety tip: Doubling your speed roughly quadruples your stopping distance - that's why speed limits exist!
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of 5
Crash Safety and Deceleration
When cars crash, deceleration (negative acceleration) creates enormous forces that can be deadly. Modern cars use crumple zones - areas designed to crumple and absorb impact energy, increasing the time it takes to stop.
The force during a collision depends on the change in momentum and the time taken. The formula F = /t shows that increasing the stopping time dramatically reduces the force experienced by passengers.
A worked example: A 1500kg car travelling at 15 m/s hits a wall and stops in 0.07 seconds. The force calculation gives -321,429N - that's equivalent to about 32 tonnes pushing backwards! The negative sign shows the force opposes the original motion.
Engineering insight: Crumple zones, airbags, and seatbelts all work by increasing stopping time, which reduces the forces acting on your body during a crash.
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Physics102 views·Updated May 29, 2026·5 pagesGCSE Physics Edexcel Topic 2: Motion Notes (Combined and Triple)
Siddharth Sriram@sidlovestransprt
Forces are everywhere around you - from walking down the street to cars braking suddenly. Understanding how forces work helps explain why objects move, stop, or stay still, and it's crucial for understanding everything from car safety to space travel.
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Forces and Newton's First Law
Every time you push, pull, or bump into something, you're applying a force, measured in Newtons (N). When multiple forces act on an object, they combine to create a resultant force - think of it as the 'winning' force after all forces battle it out.
Here's the key bit: if forces are balanced , nothing changes. If they're unbalanced, things start moving or change speed. A book sitting on your desk has balanced forces - gravity pulls down whilst the desk pushes up with equal force.
Newton's First Law tells us that objects are lazy - they don't want to change what they're doing. This laziness is called inertia, and it's why you lurch forward when a bus suddenly stops.
Quick Check: A football rolling on grass gradually slows down because friction (an external force) acts against its motion, proving Newton's First Law!
2
of 5Sign up to see the content. It's free!
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Mass, Weight, and Newton's Second Law
Don't confuse mass and weight - your mass (measured in kg) stays the same whether you're on Earth or the Moon, but your weight (measured in N) changes because weight is what happens when gravity acts on your mass.
Newton's Second Law gives us the famous equation: F = ma . This means heavier objects need more force to speed up, and lighter objects accelerate more easily with the same force.
The mathematical relationship shows that force is proportional to the rate of change in momentum. Essentially, if you want to change how fast something's moving, you need to apply force - and the bigger the change you want, the more force you'll need.
Real-world example: It takes much more force to accelerate a lorry than a bicycle because the lorry has significantly more mass!
3
of 5Sign up to see the content. It's free!
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Newton's Third Law
Newton's Third Law is probably the most misunderstood: "For every action, there is an equal and opposite reaction." This doesn't mean forces cancel out - they act on different objects entirely.
When you sit on a chair, you push down with 200N whilst the chair pushes up on you with 200N. These forces are equal in size but opposite in direction. You don't fly off because the forces balance, but if you applied more force than the chair could handle, it would break.
This law explains how rockets work in space (they push gas backwards, so the gas pushes the rocket forwards) and why you can walk (you push backwards on the ground, so the ground pushes you forwards).
Think about it: When you jump, you push down on the Earth and it pushes back up on you - but you move more because you have much less mass than the planet!
4
of 5Sign up to see the content. It's free!
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- Improve your grades
- Join milions of students
Momentum and Stopping Distances
Momentum measures how hard it is to stop a moving object. A massive lorry moving slowly can have the same momentum as a small car moving quickly, making both equally difficult to stop.
The conservation of momentum means the total momentum before a collision equals the total momentum after. When two cars crash and stick together, they share their momentum between them.
Stopping distance for cars has two parts: thinking distance (how far you travel whilst reacting) and braking distance (how far you travel whilst the brakes work). Human reaction time is typically 0.25 seconds, but distractions like phones or alcohol make this much worse.
Safety tip: Doubling your speed roughly quadruples your stopping distance - that's why speed limits exist!
5
of 5Sign up to see the content. It's free!
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- Improve your grades
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Crash Safety and Deceleration
When cars crash, deceleration (negative acceleration) creates enormous forces that can be deadly. Modern cars use crumple zones - areas designed to crumple and absorb impact energy, increasing the time it takes to stop.
The force during a collision depends on the change in momentum and the time taken. The formula F = /t shows that increasing the stopping time dramatically reduces the force experienced by passengers.
A worked example: A 1500kg car travelling at 15 m/s hits a wall and stops in 0.07 seconds. The force calculation gives -321,429N - that's equivalent to about 32 tonnes pushing backwards! The negative sign shows the force opposes the original motion.
Engineering insight: Crumple zones, airbags, and seatbelts all work by increasing stopping time, which reduces the forces acting on your body during a crash.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content: Newton's Second Law
9Most popular content in Physics
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user