Laws of Motion
Easy Overview
Ever tried pushing a heavy box? The harder you push, the faster it moves. But why does it stop when you stop pushing? Newton asked the same questions 300 years ago, and his three laws are still the foundation of everything that moves. This chapter also covers friction — that annoying force that's both a hero (you couldn't walk without it) and a villain.
Newton's First Law — The Law of Laziness
An object at rest stays at rest, and an object in motion stays in motion — unless a net external force acts on it. This property is called inertia. Heavier things have more inertia — a truck is harder to move than a bicycle. That's also why you lurch forward when a bus suddenly stops: your body wanted to keep moving.
Newton's Second Law — F = ma
The net force on an object equals mass × acceleration. Not force = mass × velocity, not force = mass × speed — specifically acceleration. F = ma also tells you direction: acceleration is in the same direction as the net force. Push something east, it accelerates east. Double the force, double the acceleration. Double the mass, half the acceleration.
Newton's Third Law — Action and Reaction
Every action has an equal and opposite reaction. When you push a wall, the wall pushes you back with the same force. You feel it, right? When you jump, you push Earth down, and Earth pushes you up. But Earth barely moves because it's so massive (remember F = ma — same F, huge m, tiny a). These forces always act on different objects.
Friction — The Grip That Slows You Down
Friction is electromagnetic force between surfaces. Static friction is what you overcome to start moving something — it's self-adjusting up to a maximum (f_s ≤ μ_s N). Once moving, kinetic friction takes over, and it's usually smaller (f_k = μ_k N). Rolling friction is even smaller — that's why wheels were invented. Friction is parallel to the surface, opposite to motion.
Free Body Diagrams — The Superpower
A free body diagram is just a sketch showing all forces acting on one object. Weight (mg) down, normal force perpendicular to surface, friction parallel to surface, tension along a string. Draw arrows, label them, pick axes, break into components, set ΣF = ma. It sounds mechanical, but it literally solves every Newton's laws problem. Don't skip the diagram.
Key Points
- •First law = law of inertia; Second law = F = ma; Third law = action-reaction pairs
- •Inertia depends on mass — more mass = more inertia
- •Friction: static (f_s ≤ μ_s N) and kinetic (f_k = μ_k N)
- •Friction always opposes relative motion between surfaces
- •Angle of repose = angle where an object just starts sliding on an incline
- •Always draw a free body diagram before solving
- •Impulse = F × Δt = change in momentum (Δp)
Practice Questions
- A 5 kg box is pushed with 20 N on a rough surface (μ_k = 0.3). Find acceleration.
- State and prove Newton's second law. Derive F = ma.
- A body of mass 2 kg is kept on a rough inclined plane (θ = 30°, μ_s = 0.6). Will it slide?
- Explain why a horse can pull a cart using Newton's laws.