Describing motion: speed, velocity and acceleration
Physics starts with describing how things move. Speed is how fast you go — distance divided by time. Velocity is speed plus a direction: 30 m/s north is a velocity, while 30 m/s is just a speed. On tests, that one word 'direction' is the whole difference between the two.
Acceleration is any change in velocity over time — speeding up, slowing down, or turning. Yes, slowing down counts: it's acceleration pointing opposite your motion. A car rounding a bend at a steady 30 m/s is still accelerating, because its direction is changing.
- Speed = distance ÷ time, measured in m/s
- Velocity = speed with a direction (30 m/s east)
- Acceleration = change in velocity ÷ time, measured in m/s²
- Slowing down and turning both count as acceleration
- Distance vs displacement: the path you took vs the straight-line change in position
A skateboarder covers 60 meters in 12 seconds. Speed = distance ÷ time = 60 ÷ 12 = 5 m/s. If she rides the same 60 m back, her distance is 120 m but her displacement is 0 — she's back where she started.
Newton's three laws, minus the headache
First law (inertia): an object keeps doing what it's doing — resting or moving in a straight line at constant speed — unless a net force acts on it. That's why you lurch forward when a bus brakes: your body was moving, and it wants to keep moving.
Second law: net force equals mass times acceleration, F = ma. Bigger force, bigger acceleration; bigger mass, smaller acceleration for the same push. Third law: forces come in pairs — for every action force there's an equal and opposite reaction force, which is why rockets fly by pushing gas down.
- 1st law: no net force → no change in motion (inertia)
- 2nd law: F = ma — force (N) = mass (kg) × acceleration (m/s²)
- 3rd law: every push is paired with an equal push back
- Mass measures 'how much stuff'; weight is the force of gravity on it
A 1,500 kg car accelerates at 2 m/s². F = ma = 1,500 × 2 = 3,000 N of net force. Notice the units: kilograms times meters per second squared gives newtons.
Net force, friction and falling
Forces are pushes and pulls, and they add. Net force is the total after combining them: equal pushes in opposite directions cancel to zero (balanced forces → constant velocity or stillness), while any leftover force (unbalanced) causes acceleration. Friction is the force that opposes sliding between surfaces — it's why a rolling ball eventually stops.
Gravity pulls every object toward Earth at about 9.8 m/s², written g. Your weight is W = mg: a 50 kg student weighs about 490 N on Earth but only about 80 N on the Moon. In a vacuum a hammer and a feather fall together, because gravity accelerates everything the same — air resistance, not mass, is what makes real falls look different.
- Balanced forces → no change in motion; unbalanced → acceleration
- Friction opposes sliding between surfaces
- Gravity accelerates falling objects at g ≈ 9.8 m/s²
- Weight = mg (a force, in newtons); mass never changes with location
- Air resistance slows real falls; in a vacuum all objects fall together
Key concepts to memorize
🎯 Study tips for this topic
- Say formulas in words every time — 'speed is distance over time' — until the symbols feel like nicknames.
- Draw every problem: a sketch with arrows for forces makes net force visible.
- Check units at the end; if the units are wrong, the answer is wrong.
- Drill F = ma with three different unknowns (F, m, a) so rearranging becomes automatic.
- Redo missed problems 24 hours later — physics skill is built from reps, not reading.
