Forces are everywhere around us - from the weight pulling...
Physics159 views·Updated 4 Sept 2026·4 pagesUnderstanding Physics: Forces - AQA Combined Science
Maddie@mads_lh
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Physics - Forces
Vectors and scalars are the foundation of forces. A vector has both size and direction (like velocity and force), whilst a scalar only has size (like speed and mass). Think of vectors as arrows - the longer the arrow, the bigger the magnitude.
Forces are simply pushes or pulls between objects. Contact forces happen when objects touch (like friction when you rub your hands together), while non-contact forces work at a distance (like gravity pulling you towards Earth).
Weight isn't the same as mass - your mass stays constant, but your weight changes depending on the gravitational field strength. On Earth, weight = mass × 9.8 N/kg. The Moon has weaker gravity, so you'd weigh less there but have the same mass.
The resultant force is what you get when you add up all forces acting on an object. If forces balance out (resultant = 0), the object either stays still or moves at constant speed. Unbalanced forces cause acceleration in the direction of the resultant force.
💡 Quick Tip: Work done = force × distance. When you push against friction, this energy becomes heat - that's why your hands warm up when you rub them together!
2
of 4
Springs and Motion Basics
Springs demonstrate how objects deform under force. Elastic deformation means the spring bounces back to its original shape - like a rubber band. Plastic deformation means it stays bent permanently, like a paperclip that's been twisted too far.
Hooke's Law shows that extension is directly proportional to applied force: F = k × e (force = spring constant × extension). This only works up to the limit of proportionality - push too hard and the spring breaks this rule.
When you stretch a spring, you're storing elastic potential energy. The energy stored equals ½ × k × e². This energy gets released when the spring returns to its original length.
Distance vs displacement and speed vs velocity are crucial distinctions. Distance and speed are scalars (no direction), while displacement and velocity are vectors (include direction). A car going round a roundabout might travel 100m distance but have zero displacement if it ends where it started.
💡 Quick Tip: Typical speeds to remember - walking , cycling , cars , and sound in air . These often appear in exam questions!
3
of 4
Graphs and Newton's Laws
Displacement-time graphs have velocity as their gradient. For curved lines, draw a tangent at any point to find the instantaneous velocity. Velocity-time graphs have acceleration as their gradient, and the area under the line gives you distance travelled.
A steeper gradient means greater acceleration, negative gradients show deceleration, and horizontal lines indicate constant speed. When the line is at zero velocity, the object is stationary.
Newton's First Law explains inertia - objects resist changes to their motion. A book on a table stays put, and a moving car keeps moving unless forces act on them. This is why you lurch forward when a car brakes suddenly.
Newton's Second Law gives us the fundamental equation: F = ma (force = mass × acceleration). Doubling the force doubles the acceleration, but doubling the mass halves the acceleration for the same force.
💡 Quick Tip: On velocity-time graphs, count the squares under the line to find distance travelled. Each square represents a specific distance based on the graph's scale.
4
of 4
Newton's Third Law and Vehicle Safety
Newton's Third Law states that every action has an equal and opposite reaction. When you walk, you push backwards on the ground, and the ground pushes you forwards. The forces are equal in size but opposite in direction.
Stopping distances involve two parts: thinking distance (how far you travel while reacting) and braking distance (how far you travel while slowing down). Stopping distance = thinking distance + braking distance.
Thinking distance increases with speed, tiredness, alcohol, drugs, and distractions. Braking distance increases with speed, wet/icy roads, worn tyres, and faulty brakes. Typical reaction times range from 0.2 to 0.9 seconds.
Momentum is always conserved in collisions and explosions. The total momentum before equals the total momentum after, assuming no external forces like friction interfere. This principle explains everything from car crashes to rocket launches.
💡 Quick Tip: Remember that momentum is a vector - direction matters! In head-on collisions, the momentums are in opposite directions, so you subtract rather than add them.
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Physics159 views·Updated 4 Sept 2026·4 pagesUnderstanding Physics: Forces - AQA Combined Science
Maddie@mads_lh
Forces are everywhere around us - from the weight pulling you down to the friction stopping you from slipping. Understanding how forces work is crucial for explaining everything from why objects fall to how cars brake safely.
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Physics - Forces
Vectors and scalars are the foundation of forces. A vector has both size and direction (like velocity and force), whilst a scalar only has size (like speed and mass). Think of vectors as arrows - the longer the arrow, the bigger the magnitude.
Forces are simply pushes or pulls between objects. Contact forces happen when objects touch (like friction when you rub your hands together), while non-contact forces work at a distance (like gravity pulling you towards Earth).
Weight isn't the same as mass - your mass stays constant, but your weight changes depending on the gravitational field strength. On Earth, weight = mass × 9.8 N/kg. The Moon has weaker gravity, so you'd weigh less there but have the same mass.
The resultant force is what you get when you add up all forces acting on an object. If forces balance out (resultant = 0), the object either stays still or moves at constant speed. Unbalanced forces cause acceleration in the direction of the resultant force.
💡 Quick Tip: Work done = force × distance. When you push against friction, this energy becomes heat - that's why your hands warm up when you rub them together!
2
of 4Sign up to see the content. It's free!
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Springs and Motion Basics
Springs demonstrate how objects deform under force. Elastic deformation means the spring bounces back to its original shape - like a rubber band. Plastic deformation means it stays bent permanently, like a paperclip that's been twisted too far.
Hooke's Law shows that extension is directly proportional to applied force: F = k × e (force = spring constant × extension). This only works up to the limit of proportionality - push too hard and the spring breaks this rule.
When you stretch a spring, you're storing elastic potential energy. The energy stored equals ½ × k × e². This energy gets released when the spring returns to its original length.
Distance vs displacement and speed vs velocity are crucial distinctions. Distance and speed are scalars (no direction), while displacement and velocity are vectors (include direction). A car going round a roundabout might travel 100m distance but have zero displacement if it ends where it started.
💡 Quick Tip: Typical speeds to remember - walking , cycling , cars , and sound in air . These often appear in exam questions!
3
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Graphs and Newton's Laws
Displacement-time graphs have velocity as their gradient. For curved lines, draw a tangent at any point to find the instantaneous velocity. Velocity-time graphs have acceleration as their gradient, and the area under the line gives you distance travelled.
A steeper gradient means greater acceleration, negative gradients show deceleration, and horizontal lines indicate constant speed. When the line is at zero velocity, the object is stationary.
Newton's First Law explains inertia - objects resist changes to their motion. A book on a table stays put, and a moving car keeps moving unless forces act on them. This is why you lurch forward when a car brakes suddenly.
Newton's Second Law gives us the fundamental equation: F = ma (force = mass × acceleration). Doubling the force doubles the acceleration, but doubling the mass halves the acceleration for the same force.
💡 Quick Tip: On velocity-time graphs, count the squares under the line to find distance travelled. Each square represents a specific distance based on the graph's scale.
4
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Newton's Third Law and Vehicle Safety
Newton's Third Law states that every action has an equal and opposite reaction. When you walk, you push backwards on the ground, and the ground pushes you forwards. The forces are equal in size but opposite in direction.
Stopping distances involve two parts: thinking distance (how far you travel while reacting) and braking distance (how far you travel while slowing down). Stopping distance = thinking distance + braking distance.
Thinking distance increases with speed, tiredness, alcohol, drugs, and distractions. Braking distance increases with speed, wet/icy roads, worn tyres, and faulty brakes. Typical reaction times range from 0.2 to 0.9 seconds.
Momentum is always conserved in collisions and explosions. The total momentum before equals the total momentum after, assuming no external forces like friction interfere. This principle explains everything from car crashes to rocket launches.
💡 Quick Tip: Remember that momentum is a vector - direction matters! In head-on collisions, the momentums are in opposite directions, so you subtract rather than add them.
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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.
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4.6/5App Store
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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
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