Forces, motion and the vector habit
Newton's second law, ΣF = m a, is a statement about *resultant* force: only the vector sum of all forces equals mass times acceleration. That single word — sum — is where most errors begin.
Start every problem by drawing a free-body diagram: one object, arrows for each force, nothing else. For a block on a ramp at angle θ you get gravity mg downward, normal force N perpendicular to the surface, and friction f along it. Resolve gravity into components mg sin θ down the slope and mg cos θ into it, and the equation along the slope becomes mg sin θ − f = ma. Note that the *same* gravity contributes with a sine and a cosine — that is what resolution does.
Two categories of quantity govern mechanics:
- Vectors — force, velocity, acceleration, momentum, torque: magnitude *and* direction; they add head-to-tail and obey Pythagoras when perpendicular.
- Scalars — mass, energy, work, speed, temperature: magnitude only; they simply add.
Speed is the magnitude of velocity, but *changes* in speed and changes in velocity are different things: an object moving in a circle at constant speed is accelerating, because the direction of its velocity keeps changing.
Uniform acceleration problems use the three kinematic equations (for acceleration a, time t, initial velocity u, final velocity v, displacement s):
- v = u + a t
- s = u t + ½ a t²
- v² = u² + 2 a s
Free fall near Earth's surface is the special case a = g ≈ 9.81 m/s², upward positive so a = −9.81. A dropped stone reaches 19.6 m/s after 2 s and falls about 19.6 m in that time — numbers worth memorising as a scale check.
Friction closes the loop: f ≤ μ N, with μₛ (static) slightly larger than μₖ (kinetic). Friction opposes *relative motion*, not motion — which is why a rolling wheel's contact patch is nearly static against the road.