Newton's First Law — Inertia
Overview
An object remains at rest or moves with constant velocity unless acted upon by a net external force. This is the law of inertia. 'Constant velocity' includes staying at rest (v = 0). Implication: if an object is not accelerating, the net force is zero — forces are balanced.
Newton's First Law — the Law of Inertia
An object remains at rest or in uniform motion (constant velocity in a straight line) UNLESS acted upon by a net external force. This is Newton's 1st Law, and it defines the concept of inertia — the tendency of an object to resist any change in its motion. The larger the mass, the greater the inertia. If you push a shopping cart, it stays in motion after you stop — until friction (a force) stops it.
Equilibrium: when forces balance
An object is in translational equilibrium when the vector sum of all forces acting on it is zero (ΣF = 0). This doesn't mean no forces act — it means all forces cancel. A book on a table: gravity pulls it down, the normal force pushes it up — net force = 0, so it stays still. A plane flying at constant velocity: thrust = drag, lift = weight — net force = 0, uniform motion.
- ⚠Thinking 'at rest' means no forces — forces can be present and balanced
- ⚠Assuming constant velocity means no forces — it means balanced forces (ΣF = 0)