Friction is the quiet opponent of every moving thing. It lets you walk and lets brakes work, yet it also wears parts down and wastes energy as heat. In air and water it has a faster growing cousin: drag, which bites harder the quicker you go.
Friction is a force that opposes the relative motion of two solid surfaces in contact. Drag (air or water resistance) is the resistive force on an object moving through a fluid. Drag increases sharply as the object's speed increases, while solid sliding friction remains mostly constant.
Toggle between a solid block scraping along the ground and a sports car cutting through the air. Increase the speed slider and watch how the world scrolls by. To maintain that steady speed, the forward driving force must perfectly match the backward resistive force.
| Friction is useful when | Friction is a nuisance when |
|---|---|
| gripping the ground so we can walk or a car can grip the road | it wears down moving parts over time |
| brakes slow a vehicle by applying friction to the wheel discs | it wastes kinetic energy by transferring it to heat |
On a moving object, friction and drag act opposite to the direction of motion, never along it. A common slip is to draw friction helping the motion. It always resists relative motion. Drag also increases with speed, so the resistive force on a fast moving car is much larger than on a slow one.
A car moves at a steady speed along a level road. Explain, in terms of friction and drag, what must be true about the forces, and what happens to the energy supplied by the engine.