What is Newton's First Law?
Newton's first law (the law of inertia) states that an object at rest stays at rest, and an object in uniform motion continues in straight-line motion with constant speed, unless acted upon by a net external force. In short: things don't change motion unless something pushes or pulls.
Everyday intuition: A cup on a table stays put until a force (a hand) moves it. A sliding puck on an almost frictionless ice continues moving until friction or a wall stops it.
Key terms
Inertia: Resistance of an object to change its motion. More mass → more inertia.
Net external force: Sum of all external pushes/pulls — only a net force changes velocity.
Simple demonstration
Mass controls how hard it is to change motion. A heavy cart needs a larger force to speed up than a light cart.
Worked examples
Example 1 — Stationary book: A book sits on a table. Gravity pulls it down, but the table exerts an equal normal force up, so net force is zero — the book stays at rest.
Example 2 — Sliding skateboard: A skateboard coasts on smooth pavement. If we neglect friction and air drag, it would travel at constant speed in a straight line.
Quick check:
- Inertia depends on mass.
- Net force ≠ 0 ⇒ acceleration.
Quiz — check your understanding
Select the best answer for each question. You can review explanations after answering.