Friction
Friction is the force that slows things down when they rub against each other.
When you slide a wooden cutting board across a dry countertop, it moves slowly and stops fast. The rough surfaces grip each other and push back against the motion. Pour a thin layer of water under the board and it glides much faster. Less contact between the surfaces means less friction slowing it down.
Explaining friction by grade level
When wind blows close to the ground, grass and trees slow it down. That drag happens because the air rubs on rough things. Up high, there is less to rub on, so wind moves faster. The rubbing force that slows it is called friction.
Projects that explore friction
Friction is the force that slows things down when surfaces rub together. You glue a thread spool over a hole in cardboard. A balloon pushes air through the spool. That air lifts the cardboard off the table so the surfaces no longer rub.
Smooth surfaces create less friction; rough surfaces create more. To measure that difference directly, you build a small ramp 6 cm high and roll a matchbox car down surfaces covered with sandpaper, wax paper, and aluminum foil. The car rolled farthest on aluminum foil, averaging 148 cm — the smooth metal creates less resistance against the wheels. Sandpaper cut that distance to about 118 cm, because its rough texture rubs harder and slows the car faster.
Friction is the force that slows things down when they rub against each other. In this experiment, you mount four anemometers on a single pole at 1, 2, 3, and 4 meters high. The higher anemometers spin faster every time, showing that wind closer to the ground is slowed more by friction.
Friction acts inside machines, not just between surfaces you can see. In a DC motor, spinning parts rub against each other, slowing them down and wasting energy as heat. To measure these internal losses, you run four DC servomotors — ranging from 30 to 100 watts — with no load, recording the voltage and current going in. Then you cut the power and time how long each motor takes to stop. Bigger motors have larger moving parts, which creates more rubbing, and the data confirms it: they lose more energy to friction.