Physics · Grades 8–11

Friction, resistance and terminal motion

Use force direction and model limitations instead of memorized slogans.

Learn the idea

Friction opposes relative sliding, or the tendency to slide, at a contact. It is not always opposite an object's overall motion.

Static friction can adjust up to a limiting value; kinetic friction models apply while sliding.

A resistive force from a fluid often increases with speed in common models. The exact relationship depends on the situation.

A falling object reaches constant velocity when drag and other upward forces balance weight, not when gravity vanishes.

At constant velocity the upward and downward forces balance.

Its net force

F = μN = 0.2 × 50 = 10 N in this model.

10 N

Static friction is best described as what?
1. A force that can adjust up to a limit
2. Always exactly μsN
3. Always zero on a stationary object
Why can a walking person's forward force from the ground be friction?
A falling object has weight 18 N downward and drag 11 N upward. Find net downward force.

A force that can adjust up to a limit
The maximum is a limit, not the actual static force in every situation.
The foot tends to push backward at contact, and ground friction can act forward on the person.
Credit relative contact motion, not the claim that friction always points backward.
7 N
18 − 11 = 7 N downward.