The Science
of Swimming
Use Archimedes’ principle to explain floating, Newton’s laws and drag to explain swimming, follow the heart and lungs through a swim, find out what cold water does to the body in the first minute, and learn what keeps pool and lake water safe.
- 00First Principlesp.2
- 01Floating & densityp.4
- 02Push & dragp.5
- 03Heart, lungs & breathp.6
- 04Cold water & the bodyp.7
- 05Water qualityp.8
- ✦Self-check · Make it · 5 Questions · Side Questp.9
You'll need: a large clear container, tap water, salt, an egg, small household objects, a measuring cup and a notebook.
Principles
A first principle is a basic truth you can build on. The science of swimming rests on four: water pushes up on everything in it, to go forward you push water back, the body needs oxygen and must not be tricked and cold water steals heat fast.
What do I already know?
What is necessary?
Everything in this module is built from four bricks. Rate yourself on each one, then check the example to see if your rating holds up.
What is the objective?
By the end, you’ll be able to explain floating with density and Archimedes’ principle, describe propulsion and drag, explain how the heart and lungs respond to swimming and why breath-holding games are dangerous, describe cold shock and hypothermia, and explain how pool and lake water are kept safe.
- 01State Archimedes’ principle.
- 02Explain why fat floats.
- 03Name 3 kinds of drag.
- 04Explain shallow-water blackout.
- 05Describe cold shock.
- 06Name 2 lake-water hazards.
The list on the left is ours. Now make it yours. What do you want out of this?
How much do you know about the science of swimming right now? Shade the boxes in pencil. You'll come back at the end (p.9) and shade it again in pen.
Why start this way? When you know what you already have (1), what you need (2) and where you're going (3), you learn faster and you notice the gaps. Every module on the Swimming Path starts with these same three questions.
Floating & density
Buoyancy and density decide what floats.
Water pushes up
Anything in water is pushed up by a force equal to the weight of the water it displaces. If that push is bigger than the object’s weight, it floats.
An object floats if it is less dense than water (about 1,000 kg/m³).
Body density
- Fat is less dense than water; muscle and bone are denser.
- Full lungs act like a float; breathing out makes you sink a little.
- Salt water is denser than fresh water, so you float more easily in the sea.
Many people barely float, which is why lifejackets matter.
Shape matters
A steel nail sinks, but a steel ship floats. Its hollow shape displaces a huge amount of water, so the upward push is bigger than its weight. A lifejacket works the same way: it adds a lot of volume for very little mass.
- Why do some people float more easily than others?
- Why can a steel ship float but a steel nail sinks?
Kitchen float test
Put an egg in a glass of tap water. Then stir in salt a spoonful at a time. When does the egg float? Explain it with density.Why do you float more easily in the sea?
Push & drag
Swimmers push water back and fight drag.
Push water back
To move forward, a swimmer pushes water backward with hands, arms and legs. The water pushes the swimmer forward (Newton’s 3rd law).
Kicking adds some push but mostly helps keep the body level.
What slows you down
- Form drag: from your shape. A long, narrow streamline cuts it.
- Friction drag: from water rubbing on skin and suits.
- Wave drag: from making waves at the surface. It grows quickly with speed.
The fastest shape
Pushing off the wall with arms squeezed over the head, hands stacked and body straight is often the fastest a swimmer goes all race. That’s why swimmers glide in a streamline after starts and turns.
- Why do swimmers shave or wear tight caps for races?
- Which stroke do you think makes the most drag?
Shape test
In a sink or tub, pull a flat hand through the water edge-first, then palm-first. Which feels harder? Which would be better for pushing, and which for gliding?Which law explains why pushing water back moves you forward?
Heart, lungs & breath
The body needs oxygen, and it can be tricked.
Heart and lungs
Swimming uses the large muscles of the arms, legs and core. The heart pumps faster to carry oxygen to them, and you must time your breathing to the stroke.
Breathing out steadily underwater (bubbles) makes breathing in at the surface quicker and calmer.
Never hyperventilate (breathe fast and deep) before swimming underwater, and never play breath-holding games. It lowers carbon dioxide, which is what makes you want to breathe, so oxygen can drop until you black out without warning, even in shallow water.
Many strong, fit swimmers have drowned this way.
A built-in response
When your face goes into cold water, your heart rate slows a little. This dive reflex saves oxygen. It’s a normal response, not a reason to stay under longer.
- Why is holding your breath underwater for a long time dangerous?
- How does your breathing change when you swim hard?
Pulse check
On land, take your pulse for 15 seconds and multiply by 4. Do 1 minute of jumping jacks and check again. Explain the change. (Don’t try this underwater.)Why is hyperventilating before an underwater swim dangerous?
Cold water & the body
Cold water attacks breathing first, then muscles.
The first minute
Falling into cold water causes a sudden gasp and fast, uncontrolled breathing. If your head is underwater, you can breathe in water. Cold shock is a major danger in the first minute.
Alberta’s lakes and rivers are cold for much of the year, even in summer.
1-10-1 (a memory aid)
- About 1 minute: get your breathing under control. Keep your head up.
- About 10 minutes of useful movement before arms and legs stop working well. Get out or hold on.
- Up to about 1 hour before hypothermia makes you unconscious, if a lifejacket keeps you up.
These are rough times; in icy water, swim failure can come sooner.
Water steals heat
Water carries heat away from the body about 25 times faster than air at the same temperature. In a lifejacket, pull your knees to your chest (the HELP position) or huddle with others to slow heat loss.
- Why can a strong swimmer still drown in cold water?
- How would you explain cold shock to a friend?
Ice-cube test
Hold an ice cube in a dry hand for 30 seconds, then hold one in a bowl of cold water with your hand in the water for 30 seconds. Which felt colder? Stop if it hurts. Explain why.Why should you wear a lifejacket even if you’re a strong swimmer?
Water quality
Chemistry and checks keep water safe.
Kept safe by chemistry
Pools add chlorine, which kills germs. Pool staff test the water often and keep it slightly basic, around pH 7.2–7.8.
A strong “chlorine smell” usually comes from chloramines, formed when chlorine reacts with sweat and urine. Shower before you swim.
Natural hazards
- Blue-green algae (cyanobacteria) can make toxins. Alberta Health Services issues advisories for affected lakes.
- Swimmer’s itch: a rash from tiny parasites in some lakes.
- Bacteria after heavy rain. Don’t swallow lake water.
Use official sources
Before swimming in an Alberta lake, check current Alberta Health Services advisories. Avoid water that looks like pea soup or has scum, and keep dogs out of it too.
- Why does pool water smell of “chlorine” when it isn’t clean enough?
- How would you decide if a lake is safe to swim in?
Advisory search
Look up current Alberta Health Services water advisories. Find one lake with an advisory. What is it for, and what does it tell people to do?What usually causes a strong pool smell?
Quick self-check
Six questions, covering every stop. Show your thinking. Answers are upside down at the bottom; no peeking till you're done.
1 · State Archimedes’ principle.
2 · Why does fat help you float?
3 · Name two kinds of drag.
4 · What is shallow-water blackout?
5 · What is cold shock?
6 · What are blue-green algae?
Re-shade your knowledge meter
How much do you know about the science of swimming now? Flip back to p.3 and shade the meter again in pen (or shade this one). Did any of the four bricks on p.2 move from "Not yet" to "Got it"?
Test what floats
Does salt change the answer?
1 · Plan
Choose 6 small household objects (a coin, a cork, an apple, a plastic lid, a key, a crayon). Predict whether each will float in tap water. Test them in a large container, then test the ones that sank again in very salty water.
- Kitchen or sink, adult OK’d
- No glass or sharp objects
- Prediction first
- Same container
2 · Results
| Object | Tap water | Salty water |
|---|---|---|
3 · Must-haves
- Predictions vs results
- Density explained
- One surprise
- One way to improve
4 · What I found
Do this at a sink or table, not in a pool or bathtub with a young child nearby. Wipe up spills so nobody slips, and don’t drink the salty water.
5 Questions
I Still Have
This module didn't answer everything. Good. Write 5 new questions it didn't answer, one of each type below. There are no wrong questions, only ones nobody's asked yet.
★ Star the one question you'd most like answered. You'll need it on the next page.
1What I'll find out
Break your big question into 2 or 3 smaller ones you could actually answer.
2Where I'll look
3How I'll share it
Who I'll share it with:
Done by: