Does friction always oppose motion?
Short answer: Friction opposes relative slipping, not motion. Your foot pushes back, so friction pushes you forward.
The trap
The wrong idea
“Friction always opposes the motion of the object.”
It sounds reasonable, and a lot of JEE aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
When you walk forward, friction from the ground on your foot points:
- Backward
- Forward
- Downward
Show the answer
B. Forward
Friction opposes relative slipping, not motion. Your foot pushes back, so friction pushes you forward.
The click
Friction opposes relative slipping (or the tendency to slip) between two surfaces in contact. That is not the same as opposing the motion of the object.
When you walk, your foot pushes backward on the ground. If the ground were frictionless, your foot would slide backward. Static friction opposes that tendency, so it acts forward on your foot. That forward friction is the external force that accelerates you forward. The same is true for a car's driven wheels: the tyres push back on the road, and friction from the road pushes the car forward.
Another example: a box resting on the floor of a truck that is speeding up. The box moves forward with the truck because static friction from the floor acts forward on the box, in the direction of its motion. Without that friction the box would slide backward relative to the truck.
Kinetic friction on a block sliding over a fixed surface does point opposite to the block's velocity, which is where the misconception comes from. The reliable test: imagine the surfaces were frictionless and ask which way they would slip relative to each other. Friction on each surface points opposite to that slipping.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.