How a combination lock works · OakCrest Academy

🔐 How a combination lock works

This lock has three number wheels, each marked 0 to 9; together they read a three-digit number from 000 to 999.
Inside each wheel is a sleeve with one slot cut through its wall, called a gate.
A locking bar runs through the center of the wheels and carries three teeth (small lugs), one beneath each wheel.
If any gate is out of line, its tooth strikes the sleeve and the bar cannot rise: every gate must be aligned at once.
When all three gates sit over the teeth, the bar and the shackle slide upward and the lock opens.
Three wheels of ten digits give 10 × 10 × 10 = 1,000 possible codes, of which exactly one opens the lock.

New words

combination: the particular sequence of digits that opens the lock
gate: a slot in a wheel's sleeve through which a tooth can pass
shackle: the U-shaped steel loop that rises out of the lock when it opens
place value: the worth of a digit determined by its position: hundreds, tens or ones
combinations: in everyday speech, all the different codes possible; here 1,000. (In mathematics a 'combination' ignores order, so a lock's code is strictly a sequence: 1-2-3 and 3-2-1 are different codes.)

🏠 Try it for real

Examine a luggage lock or bicycle lock with number wheels. Turn one wheel slowly and count its positions before it returns to where it began. Then calculate how many codes the whole lock allows.

💭 Talk about it

If a lock had four wheels instead of three, how many codes could it have, and why does adding one wheel multiply the total rather than add to it?

Now explain it back: point at each part and say what it does.