Skip to content
the grown-up commonsPlay togetherBack to room

← The Pool Deck Is a Maths Room

The Pool Deck Is a Maths Room

Four short lessons on the numbers every swim family already handles: a clock that counts to sixty, times kept to the hundredth, yards against metres, and a sport where the smaller number wins.

Spark & Whetstone · sparkwhetstone.org/learn/the-pool-deck-is-a-maths-room · CC BY-NC-SA 4.0. Answers are at the end, so you can try each question first.

Lesson 1. The clock counts to sixty

A pace clock is a big sweep hand that goes round once a minute. A coach says “ten 50s on the :40” and each swimmer leaves every forty seconds. Because the dial counts to sixty, the leaving places move round it.

On the :40 the swimmer leaves from the top, then the 40, then the 20, then the top again: three places, a triangle. On the :45 it is four places, a square. On 1:35 it is twelve places, a twelve-point star. The rule is the dial divided by what the send-off and sixty share.

Question. A set is on a 1:50 send-off. How many different places on the clock will a swimmer leave from before it repeats?

  1. 6 places
  2. 4 places
  3. 110 places
  4. 11 places

Say it back. In your own words: why does a :40 send-off draw a triangle on the pace clock?

Lesson 2. Hundredths, and a borrow of sixty

Swim times are written minutes, colon, seconds, point, hundredths: 1:05.32 is one minute, five seconds and thirty-two hundredths. The colon and the point look alike on a scoreboard, and they are not the same kind of number.

Taking one time from another trips people up. 1:05.32 take away 58.90 is not 46.42. Borrow a minute, which is sixty seconds, not a hundred: 65.32 − 58.90 = 6.42 seconds.

Question. Last month he swam 1:02.15. Today he swam 57.80. How much faster is he?

  1. 5.35 seconds
  2. 4.35 seconds
  3. 44.35 seconds

Say it back. In your own words: what goes wrong if you read 1:05.32 as 105.32?

Lesson 3. Yards and metres

Most American winter pools are 25 yards long; most of the world, and every Olympics, uses metres. A yard is 0.9144 of a metre, so a 50 in a yards pool is about 45.7 metres of swimming.

That is why a yards time is quicker than the same swim in metres, and why conversion charts are only averages: turns, pool depth and the swimmer all change the true gap. Compare a time with another from the same kind of pool.

Question. The same swimmer races a 100 in a 25-yard pool and a 100 in a 25-metre pool. How much longer is the metres race?

  1. A little over 8 metres
  2. They are the same distance
  3. A little under 4 metres, because a yard is a little longer than a metre

Say it back. In your own words: why should you compare a yards time only with another yards time?

Lesson 4. The smaller number wins

Almost everywhere else, a bigger number is better. In a race it is the other way round: faster means less, and a “drop” is good news. Percent change is measured on a number that is shrinking.

Then the World Aquatics points table turns it back: a smaller time earns more points. Saying the flip out loud — at home, on the deck — saves years of sign mistakes.

Question. A swimmer goes from 40.00 to 38.00. By what percent did the time drop?

  1. 5.3%
  2. 2%
  3. 5%

Say it back. In your own words: what is odd about a race compared with most other numbers?

At the table: The kitchen pace clock

Players: Two or more, ages 6 and up with a grown-up. You'll need: The printed Deck Clock face (or any clock with a second hand), a pencil, paper.

  1. Print a clock face from the Deck Clock, one for each person.
  2. Before anyone draws, everyone writes a guess: what shape will a :40 send-off make? A :45? A 1:35?
  3. Draw each one by jumping the pencil round the dial by the send-off, joining the dots, until you land on the top again.
  4. Compare the shapes with the guesses, and say why the :45 and the 1:35 differ.

Seat sheets and a run-through: sparkwhetstone.org/deck-clock

Answers

  1. 6 places — 1:50 is 110 seconds. 110 and 60 share 10, and 60 ÷ 10 = 6, so the hand visits six places: a hexagon.
  2. 4.35 seconds — 1:02.15 is 62.15 seconds, and 62.15 − 57.80 = 4.35. Reading the colon as a decimal point is the trap that gives 44.35.
  3. A little over 8 metres — 100 yards is 91.44 metres, so the metres race is about 8.6 metres longer. Its time is slower for that reason, not because the swim was worse.
  4. 5% — The drop is 2 seconds, and 2 ÷ 40 = 0.05. Measuring against the new time (2 ÷ 38) gives 5.3%, which answers a different question.