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Bike Shop 3-5 / Week 07 / Thursday
4/6
Week 07 · Brakes

Thursday

Wet brakes
// Friction that stops the wheel
⏱ about 15 min

Thursday: Wet Brakes

Rain taps on the shop window. Rocket wipes a wet toy wheel with his sleeve.

"Do brakes work the same in the rain?" he asks.

Raven drips a little water on the bench and slides her finger through it. "What do you notice? It feels slippery."

Nova hovers above the puddle. "Plan a fair test," she says. "Change only one thing."

Water makes things slip

Water between two surfaces can act like a slippery layer.

With rim brakes, water on the rim lowers the friction.

In wet weather, a bike with rim brakes may need twice the distance to stop.

So riders leave extra room to stop in the rain.

WET OR DRY?
  • Read the question.
  • Tap your answer.
Water gets on the rim. What happens to the friction?
A rim brake bike is in the rain. How far might it need to stop?

A fair test

In a fair test, you change only one thing.

Everything else stays the same.

Here we change only water: dry or wet.

Same eraser, same tray, same gentle push.

  1. Get a plastic tray and a rubber eraser.
  2. Push the eraser across the dry tray. Feel how it rubs.
  3. Push it three times. Notice the same thing each time?
  4. A grown-up adds a few drops of water to the tray.
  5. Push the eraser the same way three times.
  6. Record what you feel in your notebook.
Safety first
Use a plastic tray, not glass.
Wipe up spills right away so no one slips.
No eraser? Use a bottle cap or a small toy.
TryDry tray: how it feltWet tray: how it felt
1
2
3
Made-up practice numbers
Here are made-up practice numbers for a pretend toy car test.
They are not real bike numbers.
Made-up surfaceMade-up stop distance
dry ramp20 cm
wet ramp40 cm
In the made-up table, the dry stop is ____ cm.
The wet stop is ____ cm.
Fair test ideaTrue or false?
Using the same eraser on both trays is fair.?
Using a big eraser on dry and a tiny one on wet is fair.?
Using the same gentle push each time is fair.?
WHY THIS EXERCISEOnly the water should change, so we know water caused the difference.

Careful testing! Tomorrow we learn how to check that brakes work before every ride.

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