Friction (apparent weight, friction, drag)

static friction Fs

• acts on a stationary (non-sliding / not moving) body

  • Fnet = 0

• opposes motion (is equal and opposite the force of motion)

• is not constant (increases up to a maximum value) 

• can increase its magnitude to oppose motion, up to a certain value which is proportional to the Fn:

  • Fs ≤ Fsmax = µs × Fn

Fs ≤ Fsmax = µs × Fn

what is the equation to find the static friction (Fs)?

kinetic friction Fk

• acts on a sliding/moving body

  • Fnet ≠ 0

• opposes motion (is not equal and opp the force of motion bc object is still in motion) (only slows object) 

• is (roughly) constant

• has magnitude proportional to the Fn:

  • Fk = µk × Fn

Fk = µk × Fn

what is the equation to find the kinetic friction (Fk)?

µk < µs & Fk < Fsmax

Remember: its easier to push something that’s already moving:

• µk < µs

• Fk < Fsmax

coefficients of static friction (µs) & kinetic friction (µk)

• scalars

• they are not vectors (do not have a direction)

• they have no units

• they are determined experimentally

• their value depends on the materials involved