Friction opposes sliding between two surfaces
Definition
Friction
The force between two surfaces in contact that can oppose (impede) motion and, where there is sliding, produces heating.
- Solid friction acts along the surfaces, in the opposite direction to the motion or to the attempted motion.
- It arises because even surfaces that look smooth are rough on a small scale, so tiny high points catch and interlock.
- Where the surfaces slide, friction transfers kinetic energy to internal (thermal) energy, so the surfaces warm up, which is why brakes and rubbed hands get hot.
- Friction is often useful: it gives grip for walking, for tyres on a road, and for brakes to slow a wheel.
- Friction is sometimes a nuisance: it wears moving parts and wastes energy as heat, which is reduced by lubrication, smoother surfaces, or rollers and wheels.
Example
Push a heavy box across the floor and it soon stops: friction from the floor acts backwards on the box, and where it drags it warms the floor and the box slightly.
Drag opposes motion through a fluid
Definition
Drag
The resistive force on an object moving through a fluid (a liquid or a gas), acting in the opposite direction to the object's motion.
- A liquid produces drag: water resistance pushes back on a swimmer, a boat or a submarine as it moves.
- A gas produces drag too: this is air resistance, the backward force on a moving car, a cyclist or a falling object.
- Drag increases as the object moves faster, and is smaller for a streamlined shape with a small front-on area.
- Because drag acts against the motion, a steady speed is reached when the forward driving force is balanced by the drag, so the resultant force is zero.
Common Mistake
- Friction and drag do not act on their own account; they oppose motion, so their direction depends on which way the object moves.
- Drag is not the object's weight: weight acts downwards, drag acts backwards along the motion.
- Do not say friction "uses up" energy; it transfers kinetic energy to thermal energy, warming the surfaces.
Exam technique
- When a question asks for the effect of friction or drag, name the direction (opposite to motion) and the energy transfer to heat, not just "it slows the object down".
- For a falling or driven object at steady speed, say the forces are balanced so the resultant force is zero, which is where the marks are.
- Use the word drag (or air resistance) for motion through a fluid, and friction for two solid surfaces; mixing them can cost a mark.
Self review
- In which direction does friction act on a sliding object?
- What energy transfer happens when two surfaces rub together?
- Give one everyday use of friction and one drawback.
- What name is given to the drag force on an object moving through air?
- How does the drag on a car change as it speeds up?
