Physics / Unit 3: Forces and Newton's Laws
3.2 Newton's First Law
Notes
Newton's first law (law of inertia): an object at rest stays at rest, and an object in motion keeps moving at constant velocity, unless a net force acts on it.
- Inertia is resistance to a change in motion. More mass means more inertia.
- Zero net force does not mean no motion; it means constant velocity (which can be zero).
- Equilibrium: net force is zero.
- Example: passengers lurch forward when a bus brakes because their bodies keep moving forward.
Practice
A hockey puck slides across smooth ice at constant velocity. What is the net force on it?
Constant velocity means no acceleration, so the net force is zero.
Which object has the most inertia?
Inertia depends only on mass, not on speed.
A spacecraft far from any planet turns off its engines. What happens?
With no net force, its velocity does not change.
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