Things Keep Going: Inertia

Lesson 2 of 11 · 7 min · Free, no account needed

What you will learn

A moving thing keeps moving at the same speed in a straight line unless a force changes that. This is Newton's first law.
That tendency to keep doing what it is doing is called inertia.
Forces change motion; they are not needed to keep it. On Earth things stop because friction and air push against them.

Voyager 1 has flown for decades at about 17 km/s with its engine off. What keeps it going?
Same push every time. Turn the friction down, all the way to zero.
Push after each change and watch how far the puck slides.
Deep space, nothing to rub against. Fire the engine once, then let go.
A dot drops every second after the engine stops.
Moving things keep moving. This has a name.
Newton's first law: with no net force, a thing at rest stays at rest, and a moving thing keeps the same speed in a straight line.
Inertia is that tendency to keep doing what it is doing. A force is needed to change motion, not to keep it.
On Earth things stop because of friction and air. Simplified model on the rink: we turned friction down to exactly zero, which real ice never reaches.
The bus brakes hard. Where does the cup on the shelf go?
Game: space courier. Deliver the cargo with as few pushes as you can.
Up, Down, Left, Right fire a short push. Nothing slows you down, so plan your turns and your stop.
Code: in this game the ship stops dead when the engine stops. Cross out the line that does it.
One loop turn is one frame. The engine fires only in frames 0 and 1.
A puck glides at a steady speed on ice with no friction. Which forces act on it?
Drag each force that is there onto its side. Leave out the ones that are not.
Your courier route next to the cheapest one. Press Play.

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