[Leejohnston]

Physics Fundamentals — Forces, Energy, Motion & the Laws of Nature

Physics is the study of how the universe works. 
From motion and forces to energy, electricity, waves, and the structure of matter — physics explains everything from falling apples to distant galaxies.

This thread introduces the essential ideas that all physics is built on.

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1. What Is Physics?

Physics is the science of:
• forces 
• motion 
• energy 
• electricity 
• matter 
• heat 
• waves 
• space 

It aims to understand the laws that govern the universe.

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2. Forces

A force is a push or a pull.

Measured in newtons (N).

Common forces:
• weight (gravity) 
• normal contact force 
• friction 
• air resistance 
• upthrust 
• tension 
• electrical/magnetic forces 

Balanced forces: no acceleration 
Unbalanced forces: object accelerates (speed or direction changes)

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3. Newton’s Three Laws of Motion

1st Law: 
Objects stay at rest or move at constant speed unless acted on by a force.

2nd Law: 
Force = mass × acceleration 
F = ma

3rd Law: 
Every action has an equal and opposite reaction.

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4. Energy Basics

Energy cannot be created or destroyed — only transferred.

Forms of energy:
• kinetic 
• gravitational 
• elastic 
• chemical 
• thermal 
• electrical 

Important equations:

Kinetic Energy: 

Code:

KE = ½mv²

Gravitational Potential Energy: 

Code:

GPE = mgh

Work Done: 

Code:

W = Fd

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5. Motion & Graphs

Speed: distance ÷ time 
Velocity: speed with direction 
Acceleration: change in velocity ÷ time

Distance–time graphs:
• straight line up = constant speed 
• curve = acceleration 

Velocity–time graphs:
• gradient = acceleration 
• area under graph = distance

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6. Electricity Basics

Current (I): flow of charge 
Voltage (V): energy per charge 
Resistance ®: opposition to current

Ohm’s law:

Code:

V = IR

Series circuits:
• current same everywhere 
• voltage splits

Parallel circuits:
• voltage same everywhere 
• current splits

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7. Waves

Waves transfer energy without transferring matter.

Two types:
• transverse (light, water) 
• longitudinal (sound)

Wave equation:

Code:

v = fλ

v = wave speed 
f = frequency 
λ = wavelength

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8. The Particle Model of Matter

All matter is made of tiny particles.

Solids:
• fixed shape 
• particles vibrate

Liquids:
• fixed volume 
• particles slide past each other

Gases:
• no fixed shape or volume 
• particles move fast and spread out

Density:

Code:

density = mass ÷ volume

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9. Common Physics Mistakes

❌ Mixing up speed and acceleration 
✔ speed = how fast, acceleration = how speed changes

❌ Forgetting units 
✔ m, s, N, J, V, A

❌ Thinking heavier objects fall faster 
✔ only air resistance affects falling rate

❌ Confusing mass and weight 
✔ mass = kg 
✔ weight = force (N)

❌ Using the wrong graph type 
✔ motion = distance/velocity–time graphs

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10. Practice Questions

1. A car accelerates from 10 m/s to 25 m/s in 3 seconds. Find acceleration. 
2. Calculate KE of a 6 kg object moving at 4 m/s. 
3. A 1200 kg car experiences a 3000 N force. Find acceleration. 
4. Use v = fλ: wave speed 20 m/s, frequency 5 Hz → find wavelength. 
5. A circuit has V = 12 V and I = 3 A. Find resistance.

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Summary

This beginner physics guide covered:
• forces 
• motion 
• Newton’s laws 
• energy 
• electricity 
• waves 
• matter 
• key formulas 

Mastering these ideas is the first step toward understanding the universe itself.