A LITTLE LIGHTBULB MOMENT
Physics: mechanics, fluids and thermodynamics
Newton’s first law: inertia
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The couch magnet keeps moving until something pushes back
The idea behind the cue
Inertia is an object’s resistance to changes in velocity. If the net force is zero, velocity stays constant: at rest, it stays at rest; moving, it keeps moving in a straight line at constant speed.
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Short & simple
The couch magnet keeps moving until something pushes back
Acronym
IZC — Invisible Zebras Cackle.
Story
A hockey puck glides across nearly frictionless ice like it owes the rink rent. It keeps the same velocity until rough ice and a stick check finally change its motion.
Connect the pieces
- I (Invisible) → Inertia: Resistance to changes in velocity; with zero net force, velocity does not change.
- Z (Zebras) → Zero net force: Net force sums to zero, so acceleration is zero.
- C (Cackle) → Constant velocity: At rest stays at rest, and moving stays in a straight line at constant speed in an inertial frame.
- Invisible · Inertia: Resistance to changes in velocity; with zero net force, velocity does not change.
- Zebras · Zero net force: Net force sums to zero, so acceleration is zero.
- Cackle · Constant velocity: At rest stays at rest, and moving stays in a straight line at constant speed in an inertial frame.
Know the limits
Idealized in inertial frames; real motion is altered by net force and by non-inertial reference frames.
Further reading · OpenStax science textbooks ↗Check your understanding
A puck glides on nearly frictionless ice at constant velocity. Which principle best explains why it keeps moving?
A. Pascal’s principle
B. Hooke’s law
C. Newton’s first law of motion
D. Conservation of angular momentum
Reveal explanation
Newton’s first law of motion
Newton’s first law states that without a net external force, an object maintains constant velocity. The other choices describe rotation, fluid pressure transmission, and springs.