Interlude A: Static Friction Force

Section A.3

In cases of pushing the table, but the table not moving yet, shows us what?

That Newton’s first law applies

The net horizontal force equals zero

What does static friction force depend on?

The types of surfaces that are in contact

What does static friction NOT depend on?

Surface area

Speed

Generally, we find that maximum static friction force is greater than:

Kinetic friction for the same pair of contact surfaces

The static friction force is zero →

When there is no other horizontal force

Newton’s 1st law requires that no static friction force act on the block

If a small, horizontal force F_p is applied such that the block does NOT move →

Then static friction exactly balances this force

Newton’s 1st law requires F_s = -F_p

If this horizontal force is increased such that the block is on the verge of moving (nut not moving) →

Then the maximum static friction force has been reached

Friction is determined by the answer to the question:

What is the behavior of the system relative to the surface

When do you use the maximum static friction force?

When the object is on the verge of sliding

When do you use the static friction force?

When the object is NOT sliding

When do you use the kinetic friction force?

When an object is sliding

Static friction adjusts to:

Balance applied forces up to a maximum value

The maximum static friction is proportional to?

The normal force

The kinetic friction is _____ than the maximum static friction?

Smaller