PHYS 100 Chapter Notes - Chapter 5: Elastic Modulus, Shear Modulus, Ultimate Tensile Strength
10 May 2018
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Chapter 5
Friction
- Friction is a force that opposes relative motion between systems in contact
o It is parallel to the contact surface between systems and always in a direction that
opposes motion or attempted motion of the sysems relative to each other
▪ If two systems are in contact and moving relatively to one another, then
the friction is called Kinetic Friction
▪ Static Friction is greater than kinetic friction and can act between
stationary objects
o The magnitude of the frictional force has two forms
▪ Static situations (Static Friction)
• Magnitude of static Friction (Fs) is
o Fs ≤ µsN,
▪ µs is defiend as the coefficient of static friction
▪ N is the magnitude of the normal force
▪ Motion (Kinetic Friction)
• Magnitude of Kinetic Friction fk is
o Fk = µKN
▪ µk is the coefficient of kinetic friction
o Example: If weight, W = mg = (100kg)(9.80m/s2)
o 980N
• Fs(MAX) = µsN
o 0.45(980N)
o 440N
• Fk = µkN
o 0.3(980N)
o 290N
o Example: A skier with a mass of 62kg is sliding down a slope. Find the
coefficient of kinetic fraction for the skier if the friction is known to be 45N
▪ Given: Kinetic Fraction Magnitude: 45N
• Mass: 62kg
o N = w⊥
▪ mgCos25
o Fk = µKmgCos25
o µK = 45m/62kg(9.80m/s2)(0.906) = 0.082
Drag Forces
- Always opposes the motion of an object
o Proportional to some function of the velocity of the object in the fluid
o FD = ½ CρAv2
▪ C: Drag Coefficient
▪ A: Area of the object facing the fluid
▪ P: density of the fluid
o FD = bv2
▪ b: is a constant equivalent to 0.5CpA
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