Newton's First Law
Simple Explanation
Newton's first law says an object stays at rest, or keeps moving at a constant velocity in a straight line, unless a net (unbalanced) force acts on it. This resistance to a change in motion is called inertia.
Why Do We Need It?
This law explains why a passenger lurches forward when a car brakes suddenly — the passenger's body tends to keep moving at its original speed until a force (the seatbelt) acts on it.
Worked Example
Explain a tablecloth trick using inertia
A tablecloth is yanked very quickly from under dishes, and the dishes barely move. Explain why, using Newton's first law.
Why Does This Work?
Inertia isn't a force — it's simply the tendency of any mass to keep doing whatever it's already doing (staying still, or moving steadily) until something forces it to do otherwise, exactly as the first law states.
Real-Life Example
Seatbelts
A car crashes into a wall and stops suddenly.
Without a seatbelt, a passenger's body would keep moving forward at the car's original speed (their inertia) until it hit something — the seatbelt provides the force needed to slow the passenger down safely along with the car.
Practice
A ball rolling on a frictionless, perfectly flat surface with no other forces will:
EasyCommon mistake
Believing that a constant force is needed to keep something moving — in reality, a constant velocity needs ZERO net force; a force is only needed to CHANGE the motion.
Quick Review
- An object at rest stays at rest; a moving object keeps moving at constant velocity — unless a net force acts.
- Inertia is the tendency to resist a change in motion.
- Zero net force does not mean zero motion — it means constant velocity.