Newton's first law — inertia
What we will do: We will push a block across different surfaces — from rough ground to a frictionless track — and watch what happens to its motion when no net force acts on it.
What you see on screen: A block on a track, a surface switch (rough / slippery / frictionless) and a push button. With no friction the block glides on steadily forever; with friction it slows down; a bar shows the speed. Readouts: surface, net force, speed, and state of motion.
What just happened?
On the frictionless track the pushed block kept moving uniformly forever — it did not "want" to stop. Only friction brakes it.
Newton's first law (the law of inertia):
if no net force acts on a body (or the forces balance out), the body stays at rest or moves in a straight line at constant speed:
What does it debunk?
The everyday belief that "motion needs a force". A force is needed to CHANGE motion (to speed up, slow down, turn) — not to sustain it. In space (no friction) a thrown object flies on forever.
What is inertia?
It is a body's "resistance" to a change in its state of motion — inertia is measured by mass. That is why a heavy cart is harder to start and to stop than a light one.
Why you need this for the exam:
recognizing situations where forces balance out (uniform motion or rest) and telling them apart from accelerated motion. The trick: no net force = constant speed.
Remember: without a net force a body keeps its motion (rest or uniform motion). It is friction that stops it, not "nature".
Theory & formulas
Newton's first law
If no net force acts on a body (or the forces balance out), the body stays at rest or moves in a straight line at constant speed:
The role of friction
No force is needed to sustain uniform motion — the block slows down only because a friction force acts on it. Without friction it would glide on steadily forever.
Inertia
This is a property of a body: its "resistance" to a change in its state of motion. Inertia is measured by mass — the greater the mass, the harder it is to change the body's motion.
Notation: — net force, — velocity of the body.
Frequently asked questions
›What does Newton's first law say?
A body on which no net force acts stays at rest or moves in a straight line at constant speed.
›Is a force needed for motion?
No — a force is needed to change motion; uniform motion itself continues without a force (when there is no friction).
›What is inertia?
A property of a body: its "resistance" to a change in its state of motion; it is measured by mass.
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