10 friction and drag

friction

a force that acts parallel to the interface of two surfaces that are in contact, and opposes their relative motion

friction formula

f = coefficient of friction (u) x normal reaction force (R)

coefficient of friction

the ratio of the force of of friction between two bodies and the normal reaction force

magnitude of coefficient of friction

depends on the materials in contact

• steel on ice has a low coefficient of friction

• rubber sole on the ground has a high coefficient of friction

size of coefficient of friction

the greater the interaction between the molecules of the interfacing surfaces, the greater the size

• value between 0 and 1 usually but can be greater than 1

when friction coefficient is greater than one

friction force is stronger than the normal force

static friction

when a force is applied to attempt to move a stationary object over another surface

• when force is sufficient to overcome static friction object will move

dynamic friction

once the object is in motion

• coefficient of dynamic friction is usually lower than the coefficient of static friction

influence of friction on sports performance

maximising and minimising frictional influence in order to enhance performance

influence of friction examples

• sports shoes (spikes/cleats) and playing surfaces (grass, artificial surfaces, wood)

• winter sports (skiing, ice-skating)

• use of a glove

• cycling on an indoor sloping velodrome

drag

the force or forces acting to oppose the motion of an object through a fluid medium such as air or water

surface drag

as a body moves through a fluid its outer furface catches a layer of fluid nearby which slows it down compared to the fluid further away and so causing drag

minimise surface drag

changing surface to reduce interaction between surface and fluid

• shaving swimmers body to make it smooth

form drag

as an object pushes against/through fluid, the fluid pushes back against the object

minimise form drag

streamlining object, hence minimising surface area facing the direction of the motion

• cyclist adopting low profile position

wave drag

when an object moves along the surface of a fluid, some fluid is displaced to form a wave

• waves cause additional forces that oppose the forward movement of the object

minimise wave drag

avoiding motion at the interface between air and water

• swimming underwater as long as rules allow at start of race

fluid viscosity influence on drag

the greater the viscosity, the greater the friction experienced at the boundary layer, therefore the greater the drag resistance

relative velocity on drag

drag increases dramatically at speed. for an object at rest, there is no drag force. as we apply a force to an object the acceleration increases velocity and drag thats directed opposite is increased

surface size on drag

• drag depends on the pressure variation of the air around the body

• drag is directly proportional to area of the object

• doubling surface area doubles the drag

texture on drag

amount of drag depends on the surface roughness of an object

examples of factors influencing drag

• clothing for skiers, swimmers, runners, cyclists

• equipment for cycling

• body position for speed skater an swimmer

aspects involved in annotating free-body diagram

• body weight

• friction

• ground reaction force

• air resistance