KCSE 2025 Physics P1 Q12 — Inertia (the coin and card trick)
Published
The Question
“A coin sits on a smooth card placed on top of an empty beaker, with the card bridging the mouth of the beaker. The card is suddenly pulled away horizontally. State and explain the observation.”
Picture the set-up
A coin rests on a smooth card, and the card bridges the open mouth of an empty beaker. The card is then flicked away quickly in the horizontal direction. Notice the two key facts the question gives you: the card is smooth (so there is very little friction between it and the coin) and it is pulled away suddenly.
Apply the idea of inertia
Every object resists a change to its state of motion, a property called inertia. Because the coin is at rest, it has inertia of rest. When the card is snatched away suddenly there is too little time and too little friction to drag the coin along with it, so the coin barely moves sideways and stays over the mouth of the beaker.
See what force is left on the coin
Once the card has gone, there is nothing supporting the coin. The only force now acting on it is its own weight pulling it downwards, so the coin falls vertically.
State the observation
Putting the two effects together: the coin's inertia keeps it in place while the card is whipped away, and then gravity pulls it straight down. The coin therefore drops neatly into the beaker.
Final Result
The coin drops straight down into the beaker. Its inertia of rest makes it resist the sudden sideways pull of the card, so it stays put; once the card is gone its weight pulls it vertically down into the beaker.
Why this method works
This is Newton's first law in action: a body stays at rest unless a resultant force acts on it. The force that would be needed to accelerate the coin sideways with the card can only come from friction, and because the card is smooth and moves away in a fraction of a second, that friction acts for far too short a time to give the coin any real sideways speed. So the coin is effectively left hanging in mid-air over the beaker for an instant, at which point the only unbalanced force on it is gravity — and it falls straight down. The trick works precisely because the pull is sudden; if you slid the card away slowly, friction would have time to carry the coin along with it.
The same principle explains why passengers lurch forward when a bus brakes sharply — their inertia keeps them moving while the bus slows beneath them.