What is Newton's First Law?
Newton's First Law, the law of inertia, says an object keeps doing what it's already doing — staying still or moving at constant velocity — unless a net external force acts on it. It's the foundation for understanding force and motion.
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force: ΣF = 0 means zero acceleration.
- •Stays at rest
- •No net force acting
- •Inertia resists any change
- •Example: a book on a table
- •Constant velocity
- •Straight-line path
- •Zero acceleration
- •Example: a puck sliding on frictionless ice
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Step-by-step worked examples
A 2 kg book rests on a table. The table pushes up with 19.6 N and gravity pulls down 19.6 N. Is the book in equilibrium?
ΣF = 19.6 − 19.6 = 0 N Since ΣF = 0, a = 0 → the book stays at rest (Newton's first law).
A hockey puck slides on frictionless ice at 8 m/s. What is its velocity after 10 s with no forces acting?
ΣF = 0 ⇒ a = 0 Velocity stays constant: v = 8 m/s (unchanged).
A 60 N push and a 60 N friction force act on a crate in opposite directions. Does it accelerate?
ΣF = 60 − 60 = 0 N a = ΣF/m = 0 → no, it continues at constant velocity (or stays at rest).
Flashcards
Quick quiz
Q1.Newton's First Law is also known as the law of…
Q2.If the net force on an object is zero, the object…
Q3.A ball rolling on a frictionless floor forever illustrates…
Q4.Which has more inertia?
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Common mistakes
Thinking a moving object needs a constant force to keep moving. — Correct: With zero net force, it keeps moving at constant velocity forever — no force is needed to sustain motion.
Believing Newton's first law only applies to objects at rest. — Correct: It applies equally to objects moving at constant velocity in a straight line.
Confusing 'no forces' with 'net force is zero'. — Correct: Multiple forces can act and still cancel out to ΣF = 0; the law only needs the net to be zero.
Assuming heavier objects automatically move slower. — Correct: Mass affects inertia (resistance to force), not the constant velocity itself once moving with ΣF = 0.
FAQ
What is Newton's first law of motion?
An object remains at rest or moves at constant velocity unless acted on by a net external force — the law of inertia.
What is the formula for Newton's first law?
ΣF = 0 implies a = 0: zero net force means zero acceleration.
What are examples of Newton's first law?
A book resting on a table, or a hockey puck sliding at constant speed on frictionless ice.
How is Newton's first law calculated or checked?
Sum all external forces on the object; if they cancel to zero, acceleration is zero and velocity stays constant.