Friction is an important concept in physics. It’s the force that hinders two materials from sliding past each other. Friction is essential for everyday living. Imagine a world without friction: no way to drive a car on the road, no way to walk on pavement, no way to pick up that tasty sandwich. Friction may seem like an enemy to the hearty physics follower, but it’s also your friend.

Friction comes from the interaction of surface irregularities. If you introduce two surfaces that have plenty of microscopic pits and projections, you produce friction. And the harder you press those two surfaces together, the more friction you create as the irregularities interlock more and more.

Physics has plenty to say about how friction works. For example, imagine that you decide to put all your wealth into a huge gold ingot (a bar of gold), only to have someone steal your fortune. The thief applies a force to the ingot to accelerate it away as the police start after him. Thankfully, the force of friction comes to your rescue, because the thief can’t accelerate away nearly as fast as he thought — all that gold drags heavily along the ground. See the figure, which shows the forces on the gold ingot.

The forces acting on a bar of gold.
The forces acting on a bar of gold.

So if you want to get quantitative here, what would you do? You’d say that the pulling force, Fpull, minus the force due to friction, Ffriction, is equal to the net force in the x-axis direction, which gives you the acceleration in that direction: