A sprinter explodes off the starting line. Something pushes her forward, and it is not her muscles alone: muscles cannot push her body forward without something outside to push against. The only thing touching her shoes is the track.
Friction's direction comes from one question, and on movers like the sprinter the answer is a surprise.
The question is the same one every time: which way does the object slide, or try to slide, across the surface? Friction points the opposite way.
A sliding box answers it easily: it slides rightward, so friction points leftward.
Now the sprinter's shoe. The shoe is not sliding. So ask instead: if the track were slick ice, which way would the shoe slip? Backward. A driving foot on ice shoots backward, out from under the runner.
The shoe tends to slip backward across the track, so friction on the shoe points forward. That forward friction from the track is the force that drives the sprinter off the line.
A soccer player plants her shoe and pushes off toward the goal. Her shoe tends to slip backward across the grass. The grass's friction on her shoe points:
The imagined-ice test works everywhere: picture the surface suddenly slick, find the slip direction, and point friction the opposite way.
Tobias walks across a wooden floor. As he pushes off with his back foot, which way does that foot tend to slip across the floor?
A toolbox on a truck bed gets the same treatment: as the truck pulls away, the toolbox tends to slip backward across the bed, so the bed's friction on the toolbox points forward.
A mail carrier stands still on a driveway that slopes down toward the street. Her boots tend to slip downhill. The driveway's friction on her boots points:
A hiker stands on a muddy uphill slope, boots planted. Which way does the mud's friction on her boots point?
The boots are not sliding, so run the imagined-ice test: on a slick slope, the boots would slip downhill.
Friction points against the tendency to slip.
Uphill, parallel to the slope: against the boots' tendency to slip downhill.
A moving truck pulls away from a stop, and a crate on its bed moves with it, not sliding. Which way does the bed's friction on the crate point?
If the bed were slick ice, the crate would be left behind: it would slip backward across the bed.
Friction points against that tendency.
Forward, in the truck's direction of travel.
A dog takes off running across a lawn. As its paw drives against the grass, the friction on the paw points:
A delivery van pulls away from a curb, and a package on its floor moves with it without sliding. The floor's friction on the package points:
A serving cart's wheels are locked, and it slides southward across a smooth kitchen floor. The floor's friction on the cart points: