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Motion & Forces · Question 17 of 20

Inertia

Objects don't resist motion. They resist *changes* — and that one correction took humanity two thousand years to make.

Plate XIX — The card-flick trick impulse = force × time
Tug slowly and the coin rides along; snap it and the coin drops in the glass.
£glasscoin on card on glass — now flick
FIG. XIX — THE CARD-FLICK TRICK
Pull speed 4.0 m/s — a decent flick
Slow pull = coin rides along. Sharp flick = coin stays put.
Time friction gets
313ms
Coin's stubbornness
unchanged
The coin isn't glued to the card — only a little friction connects them. A slow pull gives friction lots of time to drag the coin along. A sharp flick gives it just a few thousandths of a second — not nearly enough to budge a lazy coin. So the coin stays, gravity takes over, plop.
The short answer

Things are lazy: a resting object wants to stay put and a moving one wants to keep going, until something pushes it. That stubbornness is inertia.

What's actually happening

Aristotle taught that moving things naturally slow down and stop — and everyday life agrees with him, because everyday life is full of friction. It took Galileo's ramps and Newton's first law to spot the truth underneath: left genuinely alone, a moving object keeps moving — same speed, same direction, forever. The Voyager probes have been coasting unpowered for nearly fifty years and will outlast the engineers' grandchildren. Stopping is not natural; stopping is something friction does to you.

Inertia is the name for that stubbornness, and mass is its measure. A more massive object needs more force to speed up, slow down, or swerve by the same amount. Note what inertia is not: it is not a force, and it doesn't "push back". When a bus brakes and you lurch forward, nothing threw you — the bus changed its motion and your body, briefly unattached to anything, simply kept doing what it was doing. The seatbelt's whole job is to change your motion along with the vehicle's, before the dashboard volunteers.

Once you see it, the lurch is everywhere: water flying off a shaken dog (the water keeps going when the fur reverses), ketchup finally leaving the bottle when the bottle stops mid-shake, the tablecloth trick (yank fast enough and the dishes' inertia wins over brief friction), a hammerhead tightening on its handle when you slam the handle's butt on a bench. Each is some object loyally continuing its previous motion while the world changes around it.

Try it at home The coin drop
  1. 1Lay a playing card flat over a glass and place a coin on top of the card, centred on the rim.
  2. 2Flick the card sharply sideways with your finger.
  3. 3The card flies, the coin drops cleanly into the glass. Friction had only a hundredth of a second to drag the coin along — far too little force-time to overcome the coin's inertia.