Inertia and the first law

Does motion need a push?

Narrated video. Transcript:

Push a toy car across the floor and let go. It rolls for a while, slows down and stops.

More than two thousand years ago, Aristotle said this shows something simple: to keep a body moving, something must keep pushing it.

It sounds right. A cart stops when the horse stops pulling. A bicycle stops when you stop pedalling.

But look closer. Here is the car again, now with a small arrow at the wheels pointing backwards. That arrow is friction from the floor and air.

The car does not stop because the push ran out. It stops because friction is acting against it the whole time.

Now replace the floor with smooth ice. The same push sends the car much farther, and the friction arrow is much smaller.

Less friction, longer ride. If we could remove friction completely, how far would it go? Galileo asked exactly this question.

Aristotle’s idea was wrong. A force is needed to change motion, not to keep it going. Let’s see how Galileo proved it.

Galileo’s ramps and the law of inertia

Narrated video. Transcript:

Galileo rolled smooth balls on inclined planes. Going down, a ball speeds up. Going up, it slows down.

Now join two ramps into a V. Release the ball from a height on the left. A dashed line marks that starting height.

On smooth ramps the ball climbs the right side to almost the same height. Friction makes it stop a little short.

Make the right ramp gentler. The ball still aims for the same height, so it has to roll farther along the ramp to get there.

Gentler again, and again. Each time the ball travels farther. Watch the stopping points all line up on the dashed line.

Finally make the right side flat. Now the ball can never reach its starting height, so without friction it would roll on for ever.

This is inertia: a body stays at rest, or keeps moving in a straight line at steady speed, unless something changes it.

Newton made it his first law. Every body continues in its state of rest or of uniform motion in a straight line unless compelled by an external force to change that state.

In short: if the net external force is zero, the acceleration is zero. A spaceship far from every star, with its engines off, coasts at constant velocity for ever.

Try it: Galileo’s double ramp

Interactive simulation. Things to try:

• Press Release, then make the right ramp flatter one step at a time and release again.
• With friction off, the green marks all sit on the dashed line: the ball always gets back to its starting height.
• Set the right ramp to flat. How far does the ball go?
• Turn friction on and compare the orange marks with the green ones.

Quick check

A ball rolls onto a long, perfectly smooth, level floor. There is no friction and no air resistance. What happens to the ball?

1. It slows down gradually because its push runs out
2. It keeps moving in a straight line at the same speed
3. It speeds up because nothing holds it back
4. It stops at once because no force is acting on it

Inertia on a bus

You are standing in a bus at rest. The bus suddenly starts forward. Why do you tend to fall backwards?

1. The bus pushes your upper body backwards
2. Your feet move with the bus, but your upper body tends to stay at rest
3. Air inside the bus pushes you backwards
4. Gravity pulls you backwards when the bus moves

Summary

• First law: Net F = 0 ⇒ a = 0 (v stays constant)
• Galileo’s ramps (no friction): height reached = height released

• Aristotle was wrong: a force is needed to change motion, not to keep a body moving. Things stop because of friction.
• Inertia is a body’s resistance to a change in its state of rest or uniform motion. The bigger the mass, the bigger the inertia.
• First law: every body stays at rest or in uniform motion in a straight line unless an external force makes it change.
• If the net external force is zero, the acceleration is zero, even if several forces are acting.