KS3 · Physics

Streamlining and reducing drag

Sprint through a crowd and you hit more people every second, each one harder. Turn sideways and you slip through. Moving through air is a lot like that.

Physics · Forces

Why does drag happen?

Build the web that produces drag. Each arrow means 'this changes that' (it might turn it up or turn it down; the explanation will tell you which). Join up the factors you think are linked, then compare your web with ours.

  1. Speed through the fluid
  2. Particles pushed out of the way each second
  3. How hard each particle is pushed
  4. Streamlined shape
  5. Water instead of air
  6. Drag force (outcome)
  7. Highest speed you can reach
Add a link

No links yet.

Physics · Forces

Does it make drag bigger, smaller, or no different?

What does each design choice do to drag?

Still to sort

Reduces drag (0)

Smooth, narrow, curved: air slides past.

Where the line is: Ask: does this mean less air has to be pushed out of the way?

Increases drag (0)

More air pushed out of the way. Sometimes that is exactly what you want.

Where the line is: Ask: does this mean more air is pushed aside, or trapped?

No effect on drag (0)

Not on the list of things that change it.

Where the line is: Weight and engine size are not what decides drag. Speed, shape, size and the fluid are.

10 of 10 still to sort.

Same speed each time. Pick a design choice, then pick a box.

Four helmets

Four cycling helmets are tested in the same air at the same speed. Commit to a prediction first.

Which helmet produces the smallest drag force?

Air resistance vs water resistance

AirvsWater

Same speed, two fluids. Think of the crowd again: air is a thin crowd, water is a packed crowd with its arms linked. Start with the biggest difference.

Focus

Drag at the same speed

Air

Smaller

Water

Much larger

The insight

Water has far more to push out of the way, so the same speed gives much more drag.

How close the particles are

Air

Much further apart

Water

Much closer together

What has to be overcome

Air

The particles being pushed out of the way

Water

The particles being pushed out of the way, and the forces pulling them together

Two identical balls falling, with the same pull of gravity on each

Air

Smaller drag, so more force is left over to speed the ball up: it speeds up more quickly and reaches a higher speed.

Water

Much larger drag, so less force is left over (a smaller resultant force): it speeds up more slowly, never reaches the same speed, and takes longer to fall.

Covering 100 m

Air

A sprinter

Water

A swimmer, who takes far longer

Reading speed from an arrow

Four cars of the same model each have an air-resistance arrow drawn on them. Car C's arrow is by far the longest.

What does Car C's very long air-resistance arrow tell you?

Physics · Forces

Does drag only happen to moving things?

A strong wind is blowing against a tree. The tree is not moving at all.

Which of these is closest to what you think right now?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why moving through air and water pushes back

What you need to know

  • Drag is the force on an object moving through a liquid or a gas (a fluid). In air it is called air resistance, and in water it is called water resistance.
  • Drag comes from pushing particles out of your way: when you push them, they push back on you.
  • More speed means more drag, a streamlined shape means less drag, and water gives much more drag than air.
  • Being heavier or having a bigger engine does not change the drag force.

The big picture

Drag is the push-back from the particles of a fluid (air or water) that an object has to shove out of its way. Moving faster raises it in two ways at once, a streamlined shape lowers it, and water gives far more of it than air.

Key points

1An object moving through a fluid has to push particles out of its way, and the particles push back. That push-back is the drag force.
2Moving faster pushes more particles out of the way every second and pushes each one harder. The two effects add together, so drag goes up with speed.
3Streamlined shapes are smooth, narrow and curved. Air flows over and round them and slides past instead of being pushed directly backwards, so there is less drag and a higher top speed is possible.
4A wider, taller vehicle has more drag than a normal car at the same speed. A heavier car or a bigger engine does not change the drag.
5Sometimes drag is wanted: a parachute greatly increases it, so a landing jet or a space capsule can slow down.
6At the same speed, water resistance is much larger than air resistance, because water particles are much closer together and pull on each other. That is why the highest speed in water is much lower.

Worked example

Problem

A train has a sleek, narrow body and a smooth, curved front. Explain why this lets it reach a much higher speed.

⚠ Watch out

Thinking drag depends on how heavy or powerful something is. A heavier car or a bigger engine does not change the drag force. Speed, shape, size and the fluid do, because they decide how much fluid is pushed out of the way and how hard.

🧠

Memory hook

Drag = particles pushed out of the way, pushing back. Push more of them, or push them harder, and drag grows. Weight and engine size aren't on the list. (The crowd picture breaks down on size: air's 'people' are tiny.)

✓

Check yourself

A racing cyclist crouches low and wears a smooth, curved helmet and tight lycra. In two or three sentences, explain what this does to the air and what it does to the cyclist's top speed.

Flashcards

(14)
What is drag?
The force that acts on an object moving through a liquid or a gas (a fluid) as it pushes particles out of its way.
What is the drag force called in air, and in water?
Air resistance in air, and water resistance in water.
Where does drag come from?
From pushing particles out of the way. When you push the particles, they push back on you.
Why does drag go up when you go faster?
You push more particles out of the way every second and you push each one harder. The two effects add together.
Why does air resistance fall as a rollercoaster slows at the end of the ride?
Fewer air particles are pushed aside each second, and each collision is at a lower speed, so the force is smaller.
On a car, what does the longest air-resistance arrow tell you?
That car is moving quickly through the air, because its air resistance is much larger.
Does drag only act on moving objects?
No. Moving air also causes a force on stationary objects, like wind on a tree or air turning turbine blades.
What does a streamlined shape look like, and what does it do?
Smooth, narrow and curved. It reduces drag by letting air move past more easily.
How does streamlining reduce drag?
Air flows over the top or to the sides and slides past, instead of being pushed directly backwards, so less air has to be pushed out of the way.
Why can a streamlined vehicle reach a higher top speed?
It does not have to push as much air out of the way, so there is less drag working against it.
Does a heavier car or a bigger engine change the drag force?
No. Neither affects the drag force. A wider, taller vehicle does have more drag than a normal car at the same speed.
Why is a parachute useful?
It greatly increases the drag, so a jet plane can stop in a short distance and a space capsule can slow down just before it reaches the ground.
Why is water resistance much bigger than air resistance at the same speed?
Water particles are much closer together, so many more must be pushed out of the way, and the forces pulling water particles together must also be overcome.
Why does a swimmer take far longer than a sprinter to cover 100 m?
The drag in water is much higher, so even with the same size driving force it works against the swimmer much harder and the highest speed is much slower.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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