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Newton's first law
Beginner
A body keeps its state of rest or uniform motion unless acted on by a net force.
A body at rest stays at rest and a body in motion keeps moving at constant velocity unless a net external force acts. Motion doesn't need a force; changing motion does.
A hockey puck on smooth ice keeps gliding almost unchanged.
Everyday objects stop because friction is a force, not because "motion fades".
In more detail
Newton's first law says an object stays at rest or at constant velocity unless a net external force acts on it. It defines the idea of inertial frames.
Real-world uses
- Seatbelts restrain passengers because their bodies keep moving when a car stops suddenly.
- Spacecraft coast between engine burns without needing a continuous force.
- Passengers lurch forward when a bus brakes, because their bodies try to keep moving at the old velocity.
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Frequently asked questions
Why do things slow down if no force acts?
Because friction or air resistance is acting. The net force is not zero until those forces are included.
Is the first law just a special case of the second?
It is consistent with the second law, since zero net force gives zero acceleration, but it was stated separately.
Does a moving object need a force to keep moving?
Not in the absence of friction and air drag. Newton's first law says it keeps moving at constant velocity without any net force.
Related topics in Newton's Laws & Forces
What is a force? · Contact and non-contact forces · Inertia · Newton's second law · Units of force