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Bike Shop 3-5 / Week 03 / Thursday
4/6
Week 03 · Friction: Grip and Slip

Thursday

Brakes use friction
// The rubbing force that helps a bike go and stop
⏱ about 15 min

Thursday: Brakes Use Friction

Rocket rubs his hands together very fast. "Whoa, they feel warm!"

Raven points at the bike brakes. "What do you notice about the brake pads and the rim?"

Rocket looks. "There is a tiny gap between them."

Nova hovers by the brake lever. "When the gap closes, the surfaces rub," she says. "What would that do?"

How rim brakes slow a wheel

Many bikes have rim brakes.

While you ride, the brake pads do not touch the wheel.

When the rider presses the brake lever, the pads squeeze the rim.

The pads rub on the rim. Friction slows the wheel.

The wheel slows down and then stops.

Brakes are a way people make friction bigger on purpose.

RIM BRAKES AT WORK
  • Tap a card.
  • Then tap its spot.
1First
2Next
3Then
4Last

Friction makes heat

Friction can also make heat.

Rub your palms together fast. They feel warm.

Friction wears things out too.

Over time, the soles of shoes wear thin from rubbing the ground.

BRAKE CHECK
  • Read the question.
  • Tap your answer.
While riding, do the brake pads touch the wheel?
What do the pads squeeze?
Why did Rocket's hands feel warm?
Brakes slow the wheel using the force of ____.
Rubbing your palms together makes ____.
StatementTrue or false?
Brake pads rub the rim to slow the wheel.?
Brakes make friction smaller.?
Friction can wear things out.?
WHY THIS EXERCISEBrakes press pads onto the rim to make friction and slow the wheel.
Try it
Rub your palms together fast for ten seconds. Feel the warmth.
With a grown-up holding a bike still, look for the brake pads and the rim.
No bike? Press a pencil eraser on a toy car wheel as it rolls. Watch it slow.
Safety first
Only look at the brakes. A grown-up holds the bike still.
Do not squeeze the brake lever while fingers are near the wheel.
Keep fingers away from the rim, spokes and brake pads.
For a grown-up
If you like, slowly squeeze a brake lever while your child watches from nearby.
Point out how the pads move toward the rim.
In your shop notebook, draw a rim brake. Show the pads, the rim and the gap between them.

Brilliant braking! Tomorrow we find out how to make friction smaller where it gets in the way.

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