Friction — Physics, 7–10 years
Friction is a contact force that resists sliding and helps us grip things. Physics, 7–10 years.
Surfaces resist sliding
When two surfaces touch and one tries to slide, friction acts against the sliding. Rough surfaces usually grip more, while smooth surfaces usually let things slide more easily. Friction can slow movement, but it can also give useful grip.
Why friction matters
Without enough friction, feet would slip, pencils would slide across paper and bicycle brakes would fail to slow the wheels. Sometimes friction is a problem, such as when machine parts rub and heat up. We study it to increase or reduce grip on purpose.
Testing a book
Put the same book on a wooden board. Raise one end slowly: it begins to slide when the board is 20 degrees high. Put a sheet of smooth plastic under it and try again: it slides at 12 degrees. The plastic has less friction with the book.
The common mix-up
It is tempting to call friction a force that always makes things bad because it slows objects. That is reasonable when a toy car stops sooner on a rough floor. But the same grip lets your shoes push against the ground, so you can walk instead of slipping.
Grip and smooth movement
Shoe soles and bicycle tyres have patterns to increase friction with the ground. Skis and roller skates use smooth surfaces to reduce it. Oil in an engine also reduces rubbing between moving parts, helping them last longer.
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