Newton's first law
An object keeps its velocity (including staying still) unless a net force acts on it.
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Analogy
A hockey puck on smooth ice keeps gliding until something stops it.
Example
Crate at 3 m/s on ice.Net force = 0 N.It stays at 3 m/s.
In practice
Passengers lurch forward when a bus brakes because their bodies keep moving.
Practice
Flashcards
- What does an object do when net force is zero?It keeps a constant velocity, which may be zero.
Questions people ask
Why do objects need no force to keep moving?
In space or on very smooth ice, little pushes against motion, so objects drift on. Force is needed to change velocity, not to keep it. On Earth things stop because friction and air drag act on them. Remove those forces and motion continues unchanged.
What is inertia?
Inertia is an object's resistance to changes in its motion. More mass means more inertia. It is hard to start pushing a loaded truck and hard to stop it. Inertia is not a force. It is the property that makes force necessary to change velocity.