Friction

Lesson 5 of 11 · 7 min · Free, no account needed

What you will learn

Friction is the force between surfaces that resists sliding. It points against the motion, or against the pull.
Static friction grows to match your pull, up to a limit; then the thing slips and kinetic friction is a bit smaller.
In the simple model F ≈ μN: μ depends on the two surfaces, N is how hard they press together (on a flat floor, the weight).

Sneakers grip, bowling shoes slide. Match each sole to its floor.
Pull a 2 kg block along the bench. Turn your pull up slowly.
Watch the red friction arrow and the graph.
Stack masses on the block. How hard must you pull before it slips?
The grip between surfaces has names.
Static friction: holds a thing still. It grows to match your pull, up to a limit.
Kinetic friction: acts while sliding, a bit smaller than that limit, and against the motion.
F ≈ μN. μ (mu) depends on the two surfaces; N is how hard they press together, here the weight. Simplified model: real friction also depends on dirt, water and speed.
A bowler slides on the last step. Test a rubber and a leather sole, then tap the right one.
Game: curling. Score 5 points with three stones.
Set the throw speed. While the stone slides, hold Sweep to lower the friction a little.
AI: a robot learned to push boxes in a simulator with no friction. Then it tried a real floor.
Draw the friction force while the pull on the block grows from 0 to 15 N.
Same 2 kg block: it slips at 9.8 N and slides with 5.9 N of friction.
A 2 kg block slides on a floor with μ = 0.3. What is the friction force? Use g = 9.8 m/s².
Speed round for the checkpoint: where does friction help, where does it get in the way?

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