Physics Chapter 5: Friction and Drag

Chapter 5: Friction, Drag, and Elasticity

• Friction: A force that opposes motion between surfaces in contact. It permits movement.
• Friction is parallel to the contact surface and acts in a direction opposing motion or attempted motion.
• Kinetic friction: Friction between surfaces in relative motion. For instance, a hockey puck sliding on ice.
• Static friction: Friction between stationary surfaces. Static friction is typically greater than kinetic friction.
• The harder surfaces are pressed together, the greater the friction force required to move them.
• Adhesive forces between surface molecules contribute to friction.
• Objects in motion have fewer contact points, thus requiring a reduced friction force to maintain movement.

Drag Forces

• Drag force: A force that opposes motion in a fluid (either liquid or gas)

• Experienced when moving a hand through water; the faster the movement, the greater the force.

• Drag, like friction, always opposes motion.

• Unlike friction, the drag is proportional to a function of the object's velocity in the fluid.

• Drag force depends on shape, size, velocity, and the fluid itself.

• For bigger objects (cars, bicyclists, baseballs) not moving slowly, the drag force is proportional to the square of speed.

• Mathematically, drag force (FD) is expressed as: FD = 1/2 * C * ρ * A * v²

  • C = drag coefficient
  • ρ = density of the fluid
  • A = area of the object facing the fluid
  • v = velocity of the object

• Drag forces can impact a moving object's terminal velocity, the point where the net force becomes zero.

Elasticity: Stress and Strain

• Deformation: Change in an object's shape due to force application.

• Small deformations: Objects return to original shapes when force is removed (elastic deformation).

• Hooke's Law: For small deformations, the amount of deformation is proportional to the force applied (F = k * ΔL).

  • ΔL = change in length
  • k = proportionality constant depending on shape/composition and force direction

• Stress: Ratio of force to unit area (F/A).

• Strain: Ratio of change in length to original length (ΔL/L₀).

• For small deformations, stress is proportional to strain. -Stress = Y *Strain

• Elastic Modulus (Young's Modulus): A material property that relates stress to strain in tension or compression (Y).

• For metals, springs and other structures, the elastic region to satisfy Hooke's law is larger than in brittle materials like bones.

• In materials like bones, the elastic region is small and fracture abrupt.

• The breaking stress that leads to permanent deformation or fracture is the tensile strength.

Shear Modulus

• Shear deformation involves perpendicular forces, rather than parallel forces as in tension/compression.
• Similar to tension/compression concerning their proportional relationship to deformation, the equation for this force is, Shear Deformation (Δx)= (1/S)*(F/A)*Length(L₀) where S is the Shear Modulus.

Changes in Volume: Bulk Modulus

• Solids and liquids are difficult to compress compared to gases.

• Strong electromagnetic forces in solids and liquids resist compression.

• Change in volume (ΔV) is related to the force (F), original volume (V₀), and bulk modulus (B) as: Change In Volume (ΔV) = (-1/B)*(F/A)*Volume(V₀)

• A practical application of this principle is the creation of industrial-grade diamonds via high-pressure compression.