IGCSE Physics / Forces and shape
Friction and drag
Edexcel International GCSE Physics (4PH1), chapter 2: Forces and shape, specification point 1.16. Free lesson.
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Friction and drag are forces that oppose motion, but they act in different situations. Knowing how they change helps explain grip, resistance, and the heating caused by braking.
How friction acts
Friction acts between two solid surfaces that touch. It points opposite to the way an object moves, or is trying to move. In the box example, the 60 N push acts to the right and 40 N of friction acts to the left. The weight and normal reaction are both 80 N, so they balance vertically. Friction opposes motion between touching surfaces.
How friction differs from drag
Drag is a resistive force on an object moving through a fluid, meaning a liquid or a gas. In air it is called air resistance, and in water it is water resistance. Like friction, drag opposes motion, but it acts in a different place. Friction gets bigger when surfaces are rougher or pressed together harder. Drag gets bigger at higher speeds and with a larger front area. A stationary object has no drag acting on it.
How to change friction and drag
You can reduce resistance where it wastes energy, or increase it when you need grip. Oil is a lubricant that reduces friction between moving parts in an engine. A streamlined shape reduces drag through air. By contrast, grit increases friction on an icy road, while a parachute's large area increases drag. Worn tyres have less grip on a wet road. Deep tread helps tyres keep their grip by clearing water away. Friction is needed for a car to grip the road and brake.
How to measure friction
A force meter, also called a newtonmeter, can measure the pull needed to slide a wooden block. Pulling at a slow, steady speed means the forces are balanced, so the pull equals the friction. Using the same block on different surfaces lets you compare their friction.
Where friction transfers energy
When a car brakes, the brake pads rub against the discs on its wheels. Friction transfers energy from the car's kinetic store to the thermal store of the brakes, which heat up. Energy is then transferred to the surroundings. Rubbing your hands together warms them for the same reason. Friction transfers energy to a thermal store and heats the surfaces.
What happens at steady speed
A sledge sliding across snow still has friction acting on it, even if it travels at a steady speed. At steady speed, the pull balances the friction, so the resultant force, meaning the overall force, is zero. The same idea explains why a crouching cyclist can go faster: a more streamlined shape means less air resistance opposes the motion. For a cyclist, less drag means a higher speed for the same effort.