Forces and Friction

Forces are pushes or pulls that can move objects, stop them, or change their shape. What makes forces special is that they're vectors - meaning they have both size and direction, which is crucial for calculations.

You'll measure forces using a newton balance (basically a fancy spring) in units called Newtons (N). The spring stretches more when you apply a bigger force, giving you an accurate reading.

Friction is the force that opposes movement when surfaces rub together. Think about sliding your hand across a desk - that resistance you feel is friction. It always acts in the opposite direction to movement and generates heat. Air resistance and drag are just types of friction too.

To reduce friction, you can use ball bearings (less contact area), lubricants like oil, or even air cushions like in air hockey tables. These techniques are everywhere in real life, from car engines to skateboard bearings.

Weight comes from gravity pulling on your mass. Your mass (measured in kg) stays the same whether you're on Earth or the Moon, but your weight (measured in N) changes because gravitational field strength varies. Use the equation: W = mg.

Key Point: Remember that weight and mass are different! Mass is how much "stuff" you're made of, weight is the force of gravity on that stuff.

Motion and Velocity-Time Graphs

Motion describes how objects change position over time. You can spot different types of motion instantly on velocity-time graphs - they're like a visual story of what's happening.

A horizontal line means constant velocity (steady speed), an upward slope shows constant acceleration (speeding up at a steady rate), and a downward slope indicates constant deceleration (slowing down steadily).

Acceleration measures how quickly velocity changes per second. Use the formula: a = $v-u$/t, where v is final velocity, u is initial velocity, and t is time. It's a vector, so direction matters!

The area under a velocity-time graph gives you displacement (how far you've moved from your starting point). For rectangles, it's length × height. For triangles, it's ½ × base × height.

Exam Tip: Always remember that the area under any velocity-time graph tells you displacement - this comes up constantly in exam questions!

Newton's First Law and Terminal Velocity

Newton's First Law states that objects with balanced forces (equal forces in opposite directions) will either stay still or keep moving at constant velocity. The resultant force equals zero.

Think about a skydiver falling at terminal velocity - they're moving at constant speed because air resistance upwards exactly balances their weight downwards. No resultant force means no acceleration.

When you want to lift a box, you need a minimum force equal to the box's weight. Any less and it won't budge; any more and it accelerates upwards.

Terminal velocity happens in stages: initially, gravity is stronger than air resistance, so you speed up. As you get faster, air resistance increases until it equals gravity. Then you fall at constant speed - that's terminal velocity.

Real-World Connection: Parachutes work by massively increasing air resistance, creating a much lower (and safer!) terminal velocity for skydivers.

Newton's Second Law and Work Done

Newton's Second Law deals with unbalanced forces - when there's a resultant force in one direction. Objects accelerate in the same direction as this unbalanced force, following the equation: F = ma.

The classic hammer and feather experiment shows that all objects fall at the same rate in a vacuum. On Earth, the feather falls slower only because air resistance affects it more than the hammer.

Work done measures energy transfer when a force moves something over a distance. Use the equation: E = F × d, where E is work done in joules, F is force in newtons, and d is distance in metres.

Exam questions often use "resultant force" instead of "unbalanced force" - they mean exactly the same thing, so don't get confused by the different terminology.

Memory Trick: Work = Force × Distance. Think about pushing a heavy box - the harder you push (more force) or the further you push it (more distance), the more work you've done!

Newton's Third Law

Newton's Third Law is beautifully simple: for every action, there's an equal and opposite reaction. When you push something, it pushes back with exactly the same force.

These are called Newton pairs - like when you blow up a balloon and let it go. The air pushes out of the balloon, and the balloon pushes back on the air, making it fly around the room.

Rockets work the same way: hot gas rushes out the bottom (action), and the rocket gets pushed upwards (reaction). The rocket pushes on the gas, and the gas pushes back on the rocket.

What makes rockets actually take off is that there's an unbalanced force pointing upwards - the thrust from the engines is greater than the rocket's weight.

Fun Fact: You use Newton's Third Law every time you walk! You push backwards on the ground, and the ground pushes forwards on you, propelling you forward.

Gravitational Field Strength

Gravitational field strength varies dramatically across our solar system, depending on each planet's mass. Earth's is 9.8 N/kg, but Jupiter's massive 23 N/kg would make you feel more than twice as heavy!

The Moon's weak 1.6 N/kg explains why astronauts can bounce around so easily. Meanwhile, the Sun's crushing 270 N/kg would make movement nearly impossible.

Gravitational field strength means "force per unit mass due to gravity" - essentially how many newtons of force act on each kilogram of mass. The equation is: g = F/m.

This explains why your weight changes on different planets even though your mass stays the same. More massive planets pull harder on your mass, creating more weight.

Quick Check: If you weigh 600 N on Earth $g = 9.8 N/kg$, you'd weigh about 160 N on the Moon $g = 1.6 N/kg$ - that's why Moon walking looks so bouncy!

Scalars and Vectors

Scalars have only size (magnitude) - like mass, time, speed, energy, and distance. Vectors have both size and direction - including weight, force, velocity, acceleration, and displacement.

The difference between distance and displacement is crucial: distance is how far you've travelled total, while displacement is how far you are from your starting point in a straight line.

You can calculate displacement using Pythagoras' theorem and trigonometry. If you walk 4km east then 3km north, your total distance is 7km, but your displacement is 5km at 37° northeast.

Use c² = a² + b² for the magnitude, and tan θ = opposite/adjacent for the direction. Always give your final answer with both magnitude and direction for vectors.

Exam Essential: Questions love testing the difference between distance and displacement. Remember: distance is total journey, displacement is straight-line distance from start to finish!

Projectile Motion

A projectile is any object launched into the air where only gravity acts on it - like a thrown ball or a bullet. Understanding projectile motion means thinking about horizontal and vertical movement separately.

Horizontally, there are no forces acting, so the object travels at constant speed using: v = d/t. The horizontal velocity never changes throughout the flight.

Vertically, gravity accelerates the object downwards at 9.8 m/s² using: v = u + at. If launched horizontally, the initial vertical velocity (u) starts at zero.

The key insight is that time is always the same for both horizontal and vertical calculations. The projectile spends the same amount of time moving horizontally as it does falling vertically.

Real-World Example: When you throw a ball horizontally off a cliff, it falls down while moving forward. The time it takes to hit the ground depends only on the cliff's height, not how hard you threw it!

Electromagnetic Spectrum and Space

The electromagnetic spectrum ranges from high-energy gamma rays (short wavelength, high frequency) to low-energy radio waves (long wavelength, low frequency). Visible light sits right in the middle.

Stars produce both continuous spectra (smooth bands of colour) and line spectra (specific coloured lines) that tell us what they're made of. Each element creates a unique fingerprint of spectral lines.

A light year is the distance light travels in one year - about 9.5 × 10¹⁵ metres. It's a unit of distance, not time, despite the name containing "year".

Astronomical units (AU) compare planetary distances to Earth's distance from the Sun. Mars at 1.5 AU means it's 1.5 times further from the Sun than Earth is.

Scale Check: The nearest star is 4.2 light years away - meaning light from it takes over 4 years to reach us. Space is unimaginably vast!

Instantaneous Speed

Instantaneous speed measures how fast something is moving at one exact moment - like a car's speedometer reading when it passes a speed camera, or a runner's speed crossing the finish line.

To measure this in the lab, you'll use a light gate setup with a card attached to a moving trolley. When the card breaks the light beam, a computer timer starts and stops automatically.

The equipment includes an electronic timer, ruler, and light gate connected to a computer. You measure the card's length and the time it takes to pass through the light beam.

Calculate instantaneous speed using: v = d/t, where d is the card's length and t is the time to break the beam. The shorter the card, the more "instantaneous" your measurement becomes.

Lab Tip: Use the shortest card possible for the most accurate instantaneous speed measurement - a long card gives you average speed over the card's length, not true instantaneous speed!