GCSE Forces Revision 🔬

⚡ What is a Force?

A force is a push or a pull. That's it. Everything else is just details.

💬 "A force is like peer pressure — it makes things change what they're doing. Unlike peer pressure, forces in physics are actually measurable."

👆 Contact Forces

Normal	Surface pushing back on object
Friction	Opposes motion between surfaces
Applied	Your push or pull
Air resistance	Drag through air
Tension	Pull through a rope/string

🌌 Non-Contact Forces

Gravity	Pulls masses together
Magnetic	Attracts/repels magnets & metals
Electrostatic	Charged objects attract/repel

Key point: Forces are measured in Newtons (N) and always act in a direction. They're vectors — size AND direction both matter.

📐 Free Body Diagrams

Draw an object, stick arrows on it. Longer arrow = bigger force. Sorted.

💬 "A free body diagram is just an object with arrows. If examiners wanted art, they'd mark differently. They don't. Big arrow = big force. That's the whole game."

😴 Newton's 1st Law — Inertia

Objects are lazy. They don't change what they're doing unless a force makes them.

💬 "You already know this law. It's why you stay in bed when your alarm goes off. An object at rest stays at rest. Blame Newton, not yourself."

If resultant force = 0 N →

no change in motion

This means: stationary = stays still. Moving = keeps moving at the same speed in the same direction. Forever. Unless a force acts.

Friction force: 0 N

🚗 Newton's 2nd Law — F = ma

The most important equation in this whole page. Memorise it. Seriously.

💬 "Why does a truck hitting you hurt more than a tennis ball at the same speed? Because F = ma. The truck has way more mass. Please don't test this empirically."

F = m × a

Force (N) = Mass (kg) × Acceleration (m/s²)

Force (N)

500

Mass (kg)

100

Acceleration (m/s²)

5.0

Force (N): 500

Mass (kg): 100

🤜 Newton's 3rd Law — Action & Reaction

Every force has an equal and opposite partner. No exceptions. Ever.

💬 "When you sit on a chair, the chair pushes back on you with the same force. It's not being passive-aggressive. That's just physics. The chair cannot help it."

Force A on B = −Force B on A

The forces are equal in size, opposite in direction, and act on different objects. This is why the pair never cancel out.

🚀 Rocket
Gas pushes down → rocket pushes up

🏊 Swimming
Hands push water back → water pushes swimmer forward

🦶 Walking
Foot pushes ground back → ground pushes you forward

🪑 Chair
Your weight pushes down → chair pushes up

🌍 Weight vs Mass

Your mass is how much stuff you're made of. Your weight is gravity being annoying about it.

💬 "You'd weigh 1/6th on the Moon. Same amount of you, way less gravity pulling you down. Your diet finally works — just emigrate to the Moon."

W = m × g

Weight (N) = Mass (kg) × Gravitational field strength (N/kg)

Your mass (kg)

65 kg

Mass never changes — it's the same anywhere in the universe

637 N Your weight on Earth

➡️ Resultant Forces

Add up all the forces. What's left over? That's the resultant. That's what actually matters.

💬 "Two people arguing about which direction to go. One pushes right with 100N, one pushes left with 60N. Resultant = 40N to the right. The louder one wins. In physics, at least."

⬅️ Left force (N): 40

➡️ Right force (N): 100

⬆️ Up force (N): 0

⬇️ Down force (N): 0

🪂 Terminal Velocity

Falling objects speed up until air resistance matches gravity. Then they stay at that speed.

💬 "A skydiver doesn't fall faster forever. Air resistance builds up as they speed up, until it equals gravity, and they reach terminal velocity — the speed where the universe agrees they can stop accelerating. This is why skydivers survive. Mostly."

At terminal velocity: Weight = Air resistance

Resultant force = 0 N → no acceleration

Weight (gravity)

Air resistance

Speed

The stages:

Object starts falling — weight > air resistance → accelerates
Speeds up → more air resistance builds
Air resistance = weight → resultant force = 0 → constant speed
Open parachute → air resistance suddenly >> weight → decelerates
New, slower terminal velocity reached

💥 Momentum

The heavier and faster something is, the harder it is to stop. That's momentum.

💬 "Getting hit by a truck hurts way more than getting hit by a tennis ball, even at the same speed. Because F = Δ(mv)/Δt and the truck has way more mv. You're welcome. Don't test this."

p = m × v

Momentum (kg m/s) = Mass (kg) × Velocity (m/s)

F = Δp / Δt

Force = Change in momentum / Time

Conservation of momentum: In a collision with no external forces, total momentum before = total momentum after.

🔵 Trolley A (left)

Mass: 2 kg

Speed: 4 m/s →

🔴 Trolley B (right, stationary)

Mass: 3 kg

Speed: 0 m/s (stationary)

✅ Quick Quiz

OK, prove you've been paying attention. No pressure. (There is some. P = F/A.)

1. A car of mass 800 kg accelerates at 3 m/s². What is the resultant force on it?

2. A skydiver reaches terminal velocity. What can you say about the forces acting on them?

3. A person has a mass of 70 kg. What is their weight on Earth? (g = 9.8 N/kg)

4. Newton's 3rd Law says action-reaction pairs are equal and opposite. Why don't they cancel each other out?

5. Trolley A (4 kg, 3 m/s) hits stationary trolley B (2 kg) and they stick together. What is their combined velocity after?

📋 Key Equations

W = m × gWeight (N) = mass (kg) × g (N/kg). Earth: g = 9.8

F = m × aForce (N) = mass (kg) × acceleration (m/s²)

p = m × vMomentum (kg m/s) = mass × velocity

F = Δp / ΔtForce = change in momentum ÷ time

W = F × dWork done (J) = force (N) × distance (m)

KE = ½mv²Kinetic energy (J) = ½ × mass × speed²

GPE = mghGravitational PE (J) = mass × g × height

P = F / APressure (Pa) = force (N) ÷ area (m²)

GCSE Forces 🔬