KINS 4200 Exam 3 Linear Kinetics II

friction definition

force acting at the area of contact between two surfaces in the direction opposite that of motion or motion tendency

factors affecting magnitude of friction if bow is not moving

weight of object

ground reaction force

factors affecting magnitude of friction if box is moving

weight of object

ground reaction force

direction of motion

friction

friction force depends on

forces acting perpendicular to the surfaces

• therefore, friction depends on GRF

friction formula

F_f = μN

• μ = coefficient of friction

• N = ground reaction force

static friction definition

if applied force is exerted on object/body but no movement occurs then static friction (F_s) resists the applied force

F_sMAX defintion

maximum static friction just before applied force exceeds friction

for static bodies, friction

equal to the applied force until max

kinetic friction

resultant friction from applied force exceeding friction force to create movement

kinetic friction formula

F_k = μ_kN

for bodies in motion, friction

is constant and less than maximum static friction

calculation of dynamic coefficient of friction

μ_k = F_y/F_z OR μ_k = F_x/F_z

situations where large friction is beneficial

surface of shoes to change direction

handles of rackets, bat, clubs

situations where small friction is beneficial

bottom of ski

soles of dance shoes

fluid force definition

types of force are exerted on object/body in fluid

drag definition

resist the movement of object/body

lift definition

perpendicular to drag

makes object/body travel farther in the air

drag force definition

always resists motion

F_drag = ½ C_dApv²

Coefficient of Drag (C_d)

index of how smooth and streamlined the object is

streamlining reduces C_d

magnitude of C_d depends on

shape of object

orientation of object relative to fluid flow (A)

Area Facing Flow (A)

projected frontal area of object/body

Fluid density (or viscosity) (p)

easier to walk on land, harder to walk in water

relative velocity

fluid relative to the object/body

surface drag

friction of fluid on objects surface

pressure drag

caused by pressure differential between the front and back of object

surface drag depends on

velocity of fluid relative to surface

roughness of surface

surface area of object

properties of fluid (viscosity, density)

pressure drag

dominant form of drag in human movement

creates turbulent wake

turbulent wake definition

non-uniform flow of fluid around an object

cause of pressure drag

caused by pressure differential between the front and back of object

front: high pressure

back: lower pressure

increase pressure differential = increase resistance

lift definition

force that makes objects/bodies travel farther

bernoulli’s principle

as the speed of a moving fluid increases, the internal pressure of that fluid decreases

magnus force

generation of a sidewise force on a spinning cylindrical or spherical object

only occurs if object is spinning