Friction is a force between surfaces that resists sliding or an attempt to slide. When you walk, your foot pushes backward against the ground, and the ground pushes your foot forward. The sole can grip without sliding, so the force is static friction. This is why friction can help you move. Its direction depends on how the surfaces would move relative to one another. A box sliding to the right across a stationary floor experiences friction to the left. Your foot on the ground can experience friction in the direction you are walking.

A stationary box can resist a gentle horizontal push. Static friction grows to balance your push until it reaches its limit. In a simplified example, a box presses against the floor with a normal force of 20 N. A static friction coefficient of 0.40 gives a maximum of 8 N. Once the box slides, a kinetic coefficient of 0.30 gives about 6 N of friction. A horizontal push of that size would maintain a steady speed in this model. These coefficients describe the pair of surfaces in the example. Changing the floor or box material changes the result.

Set a paperback on a smooth tray and slowly lift one end until the book starts to slide. Repeat with the same book and tray, this time with a thin, securely attached cloth over the tray. Keep the book’s starting position and orientation the same, lift slowly, and compare the angle at which it first moves. A larger angle indicates more resistance to starting in this setup. Repeat several times and keep a hand ready to catch the book. This compares the grip between two pairs of surfaces, and the angle tells you about the threshold for starting rather than the speed after release.