Newton’s First Law and Inertia
The First Law
Newton’s first law
An object remains at rest or moves with constant velocity in a straight line unless acted on by a net external force.
Net force matters
Forces can be present without changing motion if they balance exactly. What changes motion is the net force, not any single force by itself.
Inertia
Inertia is the tendency of an object to resist changes in its motion. More mass generally means more inertia.
Equilibrium
When the net force is zero, acceleration is zero. The object may be at rest or moving with constant velocity.
If the net force on an object is zero, what must its acceleration be?
By Newton’s first law, zero net force means zero acceleration.
Correct answer: Zero
What is inertia?
Inertia describes how hard it is to change an object's motion.
Correct answer: Resistance to changes in motion.
Examples of the first law
| Situation | Why motion stays unchanged or changes |
|---|---|
| Book on a table | Forces balance, so it stays at rest |
| Air hockey puck | Low friction, so it keeps moving nearly straight |
| Cruise ship moving steadily | Net force is approximately zero |
Using the first law
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1
Step 1: Ask whether the object is at rest or moving steadily.
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2
Step 2: Check whether forces balance.
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3
Step 3: If net force is zero, acceleration is zero.
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Step 4: Conclude that motion is constant unless forces change.