The idea everyone gets wrong
Push a book across a table, and it stops the moment you stop pushing. Roll a ball across the floor, and it eventually stops by itself. From everyday life, it seems obvious that moving things naturally come to a stop, and that you need to keep pushing to keep them going.
Aristotle thought so. So did almost everyone for 2,000 years. And it's completely wrong. Understanding why is the first big step into real physics.
An experiment: take away the friction
Kick a puck across different surfaces. Pay attention to what happens as the surface gets smoother and smoother.
A puck kicked across a surface: carpet, wood, ice or a perfectly frictionless surface. Choose the surface and kick speed, then press Kick. With less friction it slides farther; with none, it never stops. Readings show its speed and distance, and a graph shows speed over time.
Slide the puck
InteractiveSurface
The math, with your numbers
Try this
- →Kick the same speed on carpet, wood, then ice. How does the stopping distance change?
- →Choose 'No friction'. Will it ever stop? What would make it stop?
- →Double the kick speed on wood. Does the stopping distance double, or more?
On carpet the puck stops quickly. On wood it goes farther. On ice it goes much farther. Galileo did a version of this with balls rolling on ramps around 1600, and asked the decisive question:
If the surface were perfectly smooth — no friction at all — when would it stop?
Never. The only reason the puck slows down is the friction from the surface. Take away friction, and nothing is left to change its motion. It keeps going at the same speed, in a straight line, forever.
Newton's First Law
Isaac Newton made Galileo's insight into the first of his three laws of motion (published in 1687):
An object at rest stays at rest, and an object in motion keeps moving at the same speed in the same direction — unless a force acts on it.
This tendency to keep doing whatever you're already doing is called inertiaAn object's resistance to any change in its motion. Mass is the measure of inertia.. Everything has it. And the amount of inertia an object has is its mass: a loaded truck has a lot of inertia (hard to start, hard to stop), a shopping trolley has little.
Say it out loud: “if the total force is zero, then the velocity stays constant — and vice versa.”
Net force· measured in newtons (N)
All the forces on the object added up as vectors (Σ means 'sum of').
Velocity· measured in m/s
Speed and direction. 'Constant' includes 'zero' — staying still.
Inertia in everyday life
Make a prediction first
You're standing on a bus. It brakes suddenly. Why do you lurch forward?
Think about it, then click to reveal the answer.
Make a prediction first
You're standing on a bus. It brakes suddenly. Why do you lurch forward?
Think about it, then click to reveal the answer.
Nothing pushes you forward! Your body was moving with the bus, and by inertia it keeps moving forward when the bus slows. The bus slows down underneath you. That's exactly why seatbelts exist: they supply the force needed to slow your body down along with the car.
For a sliding object, friction produces a steady deceleration , where (“mu”) is a number describing how grippy the surfaces are. That gives a stopping distance of — notice the : double the speed and you need four times the distance to stop. This is why speed limits near schools matter so much.
Wrap-up
Key ideas to remember
- Objects don't naturally stop. Things slow down only because a force (usually friction) acts on them.
- Newton's 1st law: with zero net force, velocity stays constant — including staying at rest.
- Inertia is the resistance to changes in motion; mass measures how much inertia something has.
- Forces don't keep things moving — they change how things move.
Check your understanding
Sources & further reading
Everything in this lesson agrees with these references. They're all free to read.