Unit 10 - friction and drag

10.1 Describe friction

a force that is parallel to the interface of 2 surfaces that are in contact and oppose their relative motion.

Write the equation for value of friction

force (newtons) = coefficient of friction x normal reaction force

= coefficient of friction is dimensionless scalar quantity that can either be static or dynamic

= normal reaction force is a force exerted perpendicular to the surfaces in contact, reaction to the applied force such as the weight of the object

what is the U in the force friction equation

-       is the coefficient of friction of the surfaces (either static or dynamic)

what is the R in the force friction equation

-     R is the normal reaction force e.g., force exerted perpendicular to surfaces in contact (as a reaction to the applied force of the objects such as the weight of object)

10.2 describe the coefficient of friction

it is a dimensionless scalar quantity that is a ratio between the force of friction between two surfaces and the normal reaction force (ratio of friction x normal force)

• amount of friction between the two surfaces

• range from 0 to 1

• near 0 = slippery, minimal friction such as metal + ice

• near 1 = high friction, hard to move such as rubber on dry concrete

what is coefficient of static friction + one sporting example

• Force between two object that are stationary

• represents the force needed to overcome the static friction & start moving the object

• Ex. Between the sprinter’s shoes & the starting block

• Ex. Between soccer players’ foot & turf when planting foot to kick a shot

• Ex. Between tennis racket & hand

  *resistance to the start of motion

what is coefficient of dynamic (kinetic) friction + one sporting example

• Friction force once an object is in motion (one or both of objects are moving)

• Force of maintaining the motion once object

• Usually lower than the coefficient of static friction, as it is easier to keep an object moving then to start it moving

• Ex. Skate blade and ice when figure skating is mobile

• Ex. Between player’s uniform & dirt when sliding into base

  *resistance to continued motion and control speed  

influence/importance of high friction in sports e.g., sprinting, rock climbing, badminton, ect (advantages of high friction)

- Maintain control, stability and power

- Enhance grip, perform movement with high force and precision

- Prevent slipping at high speeds

Outline 3 ways to enhance performance by maximising friction + link to specific sport

• Sport shoes with spikes when playing field sports = explosive speed

• rubber shoes maximise grip

• basketball shoes have patterns on sole = help with agility/sudden stops and movements

• Chalk on hands when rock climbing, powerlifting, or gymnastics 

• Use of a glove when catching a ball in baseball to increase surface area and therefore increasing friction

• gold glove use leather material, enhance friction between hand and club = more secure, consistent swing = more accurate

importance/influence of low friction in sports such as skating, skiing, bowling, cycling, ect (advantages of low friction)

-       Enhance speed, smoothness and efficiency of movement as low friction reduce resistance

outline 3 ways on how to minimise friction in sports + with link to specific sport

• Sharp blades on ice skates **create lubricating layer to enhance smooth and fast movement on ice

• Polishing/waxing bowling ball and smooth bowling lane

• Waxing skis

• Smaller and smoother bike tyres in road cycling

10.5 define drag (air/water resistance)

A force acting to oppose the motion of an object through a fluid medium like air or water; a specific type of friction

• interaction between the object’s surface and fluid it is moving through

how does viscosity impact drag

= how ‘thick’ fluid is

= water is has higher viscosity than air = more drag = harder to move in

how does surface area and shape impact drag

-       Larger the surface area expose to the fluid, the more drag

∴ surface area needs to be minimised

= streamline position, stroke technique

= reduce frontal SA in cycling (crouching) and tuck body

how does surface texture impact drag

-       Smooth surfaces reduce drag, whilst rough textures create turbulence, increasing drag

= fabric mimic shark skin, tight-fitting clothing made of smooth fabric

= golf balls with dimples to create turbulent boundary layer, reduce drag

= aerodynamic helmets and skin suits in cycling

how does relative velocity (speed difference object + fluid) impact drag

-       The faster the object moves, the greater the drag force

= balance energy output with increase of drag (training)

state 3 types of drag

surface (surface texture) form (shape/SA) and wave

define surface drag + sporting example (surface texture)

·      Between the surface of an object (body) and the fluid

i.e., when swimming, the water must move around body and limbs

Ø  A thin layer of water next to the body actually sticks to it, and moves with it causing up to 30% resistance to total drag

ways to reduce surface drag using sporting examples e.g., swimming

-       Wear tight-fitting clothing made of smooth fabric e.g., swimmers wear suits that mimic shark skin and skin suits of cyclists

-       Streamlined headgear (swim caps) + aerodynamic helmets for the cyclists

-       Shave off body hair to improve their ability to feel the water and coordinate movements

++goal dimples

define form (shape) drag

·      Refers to resistive forced caused by the shape of an object or body & flow of fluid around it  

i.e., shape, smoothness and orientation of the body can all affect a cyclist’s form drag

Ø  Crouching while cycling can lower resistance by 30%, reduce frontal area exposed to air

ways to reduce form drag (link to cycling and swimming)

-       Change the frame designs on bikes

-       Maintaining a streamlined position when swimming

-       Crouching while cycling (reduce frontal area) + tuck body

swimming = streamline position + stroke technique

define wave drag (link to swimming)

·      Opposing force caused by the body or object making waves in the fluid

Ø  size of the waves will depend on how your body is positioned in the water

how to minimise wave drag (in swimming)

= position body lower in the water to reduce wave size & streamline body position to minimise wave creation

= streamline at start for as long as possible

forces to consider when annotating athlete

• Gravity – towards the earth’s centre, mass (body weight) (down)

•   Friction – parallel to contact surface (opposite to their relative motion)

• Drag (air resistance) (opposite to their relative motion)

• Normal force – perpendicular to contact surface (up) = ground reaction force (force exerted by the ground on the body in contact with)

•   Force applied (forward = in direction of where they are going)