Newton's laws of motion and momentum are fundamental concepts that...
Physics660 views·Updated 27 Jul 2026·3 pagesUnderstanding the Laws of Motion and Momentum
Emma Corden@emmacorden_rpll
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1
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Newton's Laws and Linear Momentum
Newton's laws form the foundation of how we understand motion. Newton's first law tells us that objects at rest stay at rest, and moving objects keep moving at constant speed unless something pushes or pulls them - think of a football rolling until friction stops it.
Newton's third law states that forces always come in pairs - when you push on something, it pushes back with equal strength. These action-reaction pairs are always equal and opposite, act along the same line, involve the same type of force, but affect different objects.
Linear momentum measures how hard it is to stop a moving object. The formula is simple: momentum = mass × velocity. A heavy lorry moving slowly can have the same momentum as a light car moving quickly, which is why both can cause similar damage in crashes.
💡 Quick tip: In a closed system (no external forces like friction), momentum is always conserved - the total momentum before a collision equals the total momentum after!
2
of 3
Newton's Second Law and Impulse
Newton's second law connects force to momentum changes: the net force on an object equals the rate of change of its momentum (F = ΔP/Δt). When mass stays constant, this becomes the familiar F = ma equation you've probably seen before.
Impulse is a brilliant concept that links force and time. It equals force multiplied by the time the force acts , and here's the key insight: impulse equals change in momentum. This explains why airbags work - they increase collision time, reducing the average force on your body.
For changing forces, you calculate impulse as the area under a force-time graph. In triangular impacts, this becomes I = ½bh, where the base is time and height is maximum force.
💡 Remember: When objects rebound (bounce back), the momentum change is doubled because the object reverses direction - so ΔP = 2mv rather than just mv.
3
of 3
Collisions in Two Dimensions
Real-world collisions don't always happen in straight lines - balls bounce off walls at angles, cars crash at junctions, and snooker balls scatter across tables. Two-dimensional collisions require you to consider momentum in both horizontal and vertical directions separately.
For oblique impacts against walls, momentum parallel to the wall doesn't change if speed stays constant. However, momentum perpendicular to the wall changes by -2mv sin θ, where θ is the angle of impact.
When solving 2D collision problems, use Pythagoras' theorem and trigonometry. Break the momentum into components, apply conservation of momentum to each direction, then combine the results. The maths might look intimidating, but it's just careful application of the same principles.
💡 Pro strategy: Always draw a clear diagram showing before and after velocities with their angles - it makes the vector calculations much easier to follow and reduces mistakes.
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Physics660 views·Updated 27 Jul 2026·3 pagesUnderstanding the Laws of Motion and Momentum
Emma Corden@emmacorden_rpll
Newton's laws of motion and momentum are fundamental concepts that explain how objects move and interact with each other. These principles govern everything from car crashes to rocket launches, making them essential for understanding the physical world around us.
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Newton's Laws and Linear Momentum
Newton's laws form the foundation of how we understand motion. Newton's first law tells us that objects at rest stay at rest, and moving objects keep moving at constant speed unless something pushes or pulls them - think of a football rolling until friction stops it.
Newton's third law states that forces always come in pairs - when you push on something, it pushes back with equal strength. These action-reaction pairs are always equal and opposite, act along the same line, involve the same type of force, but affect different objects.
Linear momentum measures how hard it is to stop a moving object. The formula is simple: momentum = mass × velocity. A heavy lorry moving slowly can have the same momentum as a light car moving quickly, which is why both can cause similar damage in crashes.
💡 Quick tip: In a closed system (no external forces like friction), momentum is always conserved - the total momentum before a collision equals the total momentum after!
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Newton's Second Law and Impulse
Newton's second law connects force to momentum changes: the net force on an object equals the rate of change of its momentum (F = ΔP/Δt). When mass stays constant, this becomes the familiar F = ma equation you've probably seen before.
Impulse is a brilliant concept that links force and time. It equals force multiplied by the time the force acts , and here's the key insight: impulse equals change in momentum. This explains why airbags work - they increase collision time, reducing the average force on your body.
For changing forces, you calculate impulse as the area under a force-time graph. In triangular impacts, this becomes I = ½bh, where the base is time and height is maximum force.
💡 Remember: When objects rebound (bounce back), the momentum change is doubled because the object reverses direction - so ΔP = 2mv rather than just mv.
3
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Collisions in Two Dimensions
Real-world collisions don't always happen in straight lines - balls bounce off walls at angles, cars crash at junctions, and snooker balls scatter across tables. Two-dimensional collisions require you to consider momentum in both horizontal and vertical directions separately.
For oblique impacts against walls, momentum parallel to the wall doesn't change if speed stays constant. However, momentum perpendicular to the wall changes by -2mv sin θ, where θ is the angle of impact.
When solving 2D collision problems, use Pythagoras' theorem and trigonometry. Break the momentum into components, apply conservation of momentum to each direction, then combine the results. The maths might look intimidating, but it's just careful application of the same principles.
💡 Pro strategy: Always draw a clear diagram showing before and after velocities with their angles - it makes the vector calculations much easier to follow and reduces mistakes.
We thought you’d never ask...
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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