What are some everyday examples of friction? (For example, we couldn’t walk without friction.)
Before giving some examples, I’ll note that there are two different types of friction. First, there’s the static friction between two surfaces that are pressed together but are not sliding across one another. Second, there’s the sliding or dynamic friction between two surfaces that are moving across one another. Static friction allows objects to push one another sideways but doesn’t create thermal energy. Sliding friction also creates thermal energy (or heat).
Your example of walking is a case of static friction: your feet push backward on the sidewalk and the sidewalk reacts by pushing your feet (and you) forward. As further examples of static friction: holding a pencil, screwing in a light bulb, pulling a rope toward you hand over hand, pedaling a bicycle so that the ground pushes the wheel forward, keeping the dishes and silverware from blowing off a level picnic table on a windy day…
As examples of sliding friction: skidding the wheels of a automobile during a rapid start or stop, sliding down the pole in a fire station, skiing or skating, squeezing a bicycle’s caliper brakes against the wheel rims, shaping metal with a grinding wheel, sharpening a knife, sanding a wooden desktop…