Newton's First Law
Also known as: law of inertia, N1
Grade 9-12
View on concept mapAn object at rest stays at rest, and an object in motion continues moving at constant velocity in a straight line, unless acted upon by. Newton's first law overturned the ancient belief that objects need a force to keep moving.
Definition
An object at rest stays at rest, and an object in motion continues moving at constant velocity in a straight line, unless acted upon by.
💡 Intuition
Things keep doing what they're doing unless something pushes or pulls them.
🎯 Core Idea
Motion doesn't require a force—changing motion requires a force.
Example
Notation
\vec{F}_{\text{net}} is the vector sum of all forces on the object. \vec{v} is the velocity vector. An inertial frame is one in which this law holds without fictitious forces.
🌟 Why It Matters
Newton's first law overturned the ancient belief that objects need a force to keep moving. It explains why passengers lurch forward when a car brakes, why seatbelts are necessary, and why spacecraft travel indefinitely without fuel in the vacuum of space.
💭 Hint When Stuck
When applying Newton's first law, first determine whether the net force on the object is zero. If the net force is zero, the object either remains at rest or continues at constant velocity. If the object is accelerating or decelerating, a net force must be acting — identify it.
Formal View
🚧 Common Stuck Point
Objects on Earth slow down due to friction, not because motion naturally stops.
⚠️ Common Mistakes
- Believing that a moving object always needs a force to keep moving — constant velocity requires zero net force, not a continuous push.
- Forgetting that 'constant velocity' includes direction — changing direction (even at constant speed) requires a net force.
- Confusing Newton's first law with the second law — the first law establishes when motion changes (nonzero net force), the second law quantifies how much.
Frequently Asked Questions
What is Newton's First Law in Physics?
An object at rest stays at rest, and an object in motion continues moving at constant velocity in a straight line, unless acted upon by.
When do you use Newton's First Law?
When applying Newton's first law, first determine whether the net force on the object is zero. If the net force is zero, the object either remains at rest or continues at constant velocity. If the object is accelerating or decelerating, a net force must be acting — identify it.
What do students usually get wrong about Newton's First Law?
Objects on Earth slow down due to friction, not because motion naturally stops.
Cross-Subject Connections
🧪 Visualization Static
Visual demonstration of this concept.