Chapter Summary
This chapter is really two connected stories. Story one is about why things speed up, slow down, or stay still — that's Newton's Laws. Story two is about what happens when things collide or push apart — that's momentum. Here's the whole chapter at a glance:
Newton's First Law
No resultant force = no change in velocity. Balanced forces mean constant speed (or rest).
Newton's Second Law
Resultant force causes acceleration: ΣF = ma. Bigger force or smaller mass = bigger acceleration.
Terminal Velocity
Drag force grows until it balances the driving force — then acceleration becomes zero.
Mass, Weight & g
Mass is fixed; weight (W = mg) depends on gravitational field strength.
Core Practical 1
Three ways to measure g experimentally: electromagnet drop, light gates, and ramp & trolley.
Newton's Third Law
Every force has an equal, opposite, same-type reaction force on a different object.
Momentum
p = mv. A vector — direction matters, so momentum can be negative.
Conservation of Momentum
In a closed system, total momentum before a collision = total momentum after.
1. Force & Acceleration
Newton's First Law of Motion
Here's the core idea, and it trips people up because it feels wrong at first: objects don't need a force to keep moving. Your instinct says "if nothing's pushing it, it should stop" — but that's only because in real life, friction is almost always secretly pushing back. Newton's First Law says that if the forces on an object are perfectly balanced (including zero forces at all), the object just keeps doing whatever it was already doing — staying still, or cruising at constant velocity in a straight line.
"Resultant force" just means the overall force left over once you've added up everything acting on the object (accounting for direction). If forces-left = forces-right AND forces-up = forces-down, the resultant is zero, and the object's velocity is locked in — whatever it was.
Worked Example
A car moves at constant velocity. The driving force from the engine is 6 kN. What is the frictional force F?
Step 1 — spot the clue word: "constant velocity" means forces are balanced.
Step 2 — apply ΣF = 0: so F (friction) = D (driving force).
Step 3 — answer with units: F = 6 kN.
Practice Question
A book sits still on a table. Explain, using Newton's First Law, why the book doesn't accelerate even though gravity is pulling down on it.
Newton's Second Law of Motion
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2. Mass, Weight & Gravitational Field Strength
5. Momentum