Rocket drags a heavy box of books across the floor. "Ugh, it fights me!" he groans.
Raven rolls three round pencils under a thick book. She pushes it with one finger.
"What do you notice?" she asks. "It rolls so easily!"
Nova hovers near the pencils. "Rolling and sliding feel different," she says. "Which one rubs less?"
Sometimes friction gets in the way and wastes effort.
Oil or grease makes friction smaller. It is called a lubricant.
A lubricant makes a thin, slippery layer between moving parts.
A few drops of oil help a squeaky door hinge move smoothly.
Bike mechanics use grease between moving parts.
A bike chain gets checked and oiled regularly.
It is easier to roll something than to slide it.
Rolling friction is smaller than sliding friction.
That is why wheels are so useful.
Bike hubs have tiny balls inside called ball bearings.
They roll so the wheel turns easily.
Friction can never be made zero. No surface is perfectly smooth.
Friction is helpful in some places and unhelpful in others.
| On a bike, friction here is... | Helpful? (T) or in the way? (F) |
|---|---|
| tires gripping the ground | ? |
| brake pads squeezing the rim | ? |
| a dry, squeaky chain rubbing the gears | ? |
| a wheel hub that rubs instead of rolling | ? |
These are made-up practice numbers, not real results.
A toy car rolled down the same ramp onto four surfaces.
| Surface (made-up practice) | How far it rolled |
|---|---|
| bare floor | 90 cm |
| paper | 75 cm |
| cloth | 40 cm |
| towel | 15 cm |
Wonderful work this week! Tomorrow is Bike Shop Day: the family ramp test.