Chapter 5: Friction, Drag, and Elasticity Flashcards
Define deformation
Change in shape due to the application of force.
Define drag force in words.
The drag force is found to be proportional to the square of the speed of the object; mathematically
What is the equation for drag force?
F_D∝v2 or F_D=1/2CρAv^2
F_D = Drag force (_ = subscript)
v = velocity
C = drag coefficient
A = area of the object facing the fluid
p = density of the fluid
∝ = directly proportional
Define friction
A force that opposes relative motion or attempts at motion between systems in contact
Define Hooke’s Law in works
Proportional relationship between the force on a material and the deformation it causes
What is Hooke’s law in a mathematical expression?
F = kΔL
F = force
k= spring constant
ΔL = amount of deformation (change in length)
Define kinetic friction
A force that opposes the motion of two systems that are in contact and moving relative to one another
What is the equation for the magnitude of kinetic friction?
fk=μkN
fk = force of kinetic friction
uk = coefficient of kinetic friction
N = normal force
What is the equation for the “magnitude of static friction”
fs≤μsN
fs = force of static friction
us =the coefficient of static firction
N = magnitude of normal force
Define shear deformation
Deformation perpendicular to the original length of an object
Define static friction
A force that opposes the motion of two systems that are in constant and are not moving relative to one another
Define Stoke’s Law (equation)
Fs=6πrηv
r = radius of the object
η = is the viscosity of the fluid
v = velocity
Define strain
Ratio of change in length to original length
Define stress
Ratio of force to area
Define tensile strength
The breaking stress that will cause permanent deformation of fraction of a metal
Between the 3 drag force equation. Which is used for small objects, and which is used for big objects?
F_D∝v2
F_D=1/2CρAv^2
Fs = 6πηrv
Any object not in fluid: F_D∝v2
Big objects in fluid (air can be fluid): F_D=1/2CρAv^2
Small objects (such as bacteria in water): Fs = 6πηrv
T/F: Stokes’ Law is a drag force equation
True
How can the relationship between the deformation (ΔL) and the applied force (F) be written as other than Hooke’s Law?
ΔL=1/Y * F/A * L0
ΔL = deformation ( change in length)
Y = Young’s modulus depends on the substance
A = cross-sectional area
F = force
L0 = original length
What does the “F/A” in ΔL=1/Y * F/A * L0 represent?
F/A is defined as “stress” and its unit is N/m^2
What does ΔL/ L0 mean in ΔL=1/Y * F/A * L0
ΔL/L0 is defined as strain (has no unit)
What is one way to calculate stress?
Stress = Y * strain
Y = young’s modulus, depends on the substance
Strain = ΔL/L0 (change in length / original length)
What is the expression for shear deformation?
Δx=1/S * F/A *L0
S = shear modulus
F = force applied perpendicular to L0
A = cross-sectional area
What is the relationship of change in volume to other physical quantities equation?
ΔV=1/B * F/A * V0
B = bulk modulus
V0 = original volume
F/A = force per unit area
Albert pushes a 12kg box across a level floor. He pushes with a force of 43 N and observes that the box accelerates toward his push at a rate of 3 m/s^2.
What is the net force acting on the box?
36 N
Explanation:
I. Newton’s 2nd law of motion (discussed in Ch.4) says Fnet = ma
II. Net force = mass x acceleration -> F = 12kg x 3 m/s^2
III. F = 36 N (kgm/s^2 = N)