Shove a cardboard box across a gym floor and let go. It slides, slows, and stops. While it slowed, some force was acting on it along the floor. Which force, and which way did it point?
A surface exerts more than its square-on push.
A box dragged rightward across a floor feels a force from the floor pointing leftward, against the slide. That force is friction, the force you met in earlier grades as the resistance between sliding surfaces. Here is its full job description.
Friction is exerted by a surface, like the normal force. But the two point differently: the normal force is the surface's perpendicular push, and friction acts parallel to the surface.
Friction's direction, parallel to the surface, is not random: it points against the sliding of the object across the surface.
A hockey puck slides eastward across the ice. The ice's friction on the puck points:
A plastic bin slides across a garage floor. Which force on the bin is friction?
Sliding does not even have to start. Push a loaded dresser that refuses to budge, and friction acts against the attempted sliding, matching your push direction in reverse.
Mira pushes a heavy stone planter, and it does not budge. Does friction act on the planter?
So friction's recipe: find which way the object slides, or tries to slide, across the surface. Friction from that surface points the opposite way, parallel to the surface. One boundary: friction never points perpendicular to the surface. The perpendicular job belongs to the normal force alone.
And the gym box from the start: while it slid, the floor's friction pointed against its slide, parallel to the floor.
A library book slides leftward across a counter after a push. Which way does the counter's friction on the book point?
Friction points against the sliding, parallel to the surface.
The book slides leftward across the counter.
Rightward, parallel to the counter: against the slide.
Jax pushes a parked dumpster northward, and it does not move. Which way does the pavement's friction on the dumpster point?
Friction also acts against attempted sliding.
The dumpster tries to slide northward across the pavement.
Southward, parallel to the pavement: against the attempted slide.
A suitcase is dragged northward across an airport's tile floor. The floor's friction on the suitcase points:
Which statement is true of the friction a surface exerts on an object?
Tobias shoves a loaded bookcase toward the window, and it stays put. The floor's friction on the bookcase points: