Flashcards Chapter 5

What is friction?

A force that opposes motion between surfaces in contact

When does kinetic friction occur?

When one or both surfaces are sliding

What is the formula for kinetic friction?

f_k = \mu_kF_N

What does \mu_k​ represent?

The coefficient of kinetic friction

When does static friction occur?

When the surfaces are not moving relative to each other

What is the inequality that defines static friction?

f_s \le \mu_sF_N

When is static friction at its maximum value?

Right before sliding begins: f_{s}=\mu_{s}F_{N}

How do you decide which type of friction to use?

If the object isn’t moving, use static friction; use kinetic friction only once sliding begins

What direction does friction act?

Opposite the direction of actual or intended motion

What is drag force?

A force that opposes the motion of an object moving through a fluid

What does drag force always do?

It resists motion; it always acts opposite to the direction of motion

What is the formula for drag force for larger or fast-moving objects?

F_D = ½ C\rho A v²

What does each variable in the drag force formula represent?

• C: drag coefficient

• ρ: fluid density

• A: cross-sectional area facing the fluid

• v: velocity of the object

When is the drag force formula less accurate?

For very small objects, slow motion, or dense/viscous fluids (like oil or honey)

What is terminal velocity?

The constant speed when F_D = mg, so net force = 0 and acceleration stops

What happens to acceleration at terminal velocity?

Acceleration becomes zero; the object falls at constant speed

What is Stokes’ Law used for?

Small objects moving through highly viscous fluids

What is the formula for Stokes’ Law?

F_S = 6\pi r \eta v

What does each variable in Stokes’ Law represent?

• r: radius of the object

• η: fluid viscosity

• v: velocity of the object

What is the key difference between the drag force formula and Stokes’ Law?

• The drag force formula F_D = \frac{1}{2} C \rho A v^2 applies to large, fast-moving objects in fluids like air

• Stokes’ Law F_S = 6 \pi r \eta v applies to small, slow-moving objects in highly viscous fluids