Newton's first law
An object stays still or keeps moving at a steady speed in a straight line unless a resultant force acts on it.
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Analogy
A puck sliding on ice keeps going because almost nothing slows it.
In practice
This is why passengers lurch forward when a car brakes suddenly: their bodies keep moving.
Practice
Flashcards
- What does Newton's first law say?With no resultant force, an object stays still or moves at a steady speed in a straight line.
Interview questions
- Explain why a falling skydiver reaches a terminal velocity.
- Weight stays constant and pulls down
- Drag increases as speed increases
- When drag equals weight the resultant force is zero
- By Newton's first law, speed then stays constant
Questions people ask
Why do I lurch forward when a bus brakes?
Your body wants to keep moving at the same speed, as Newton's first law says. The bus slows down because of the brakes, but nothing slows you until your seat belt or the seat in front acts on you. Your body carries on forward until a force stops it.
Why do moving objects on Earth eventually stop?
Friction and air resistance are forces that act against the motion. They give a resultant force that slows the object down. In space there is almost nothing to cause such forces, so a spacecraft can keep gliding at a steady speed for years without using fuel.