Topic 10

Question 1

Describe the coefficient of
friction

Answer 1

The coefficient of friction (COF, μ) is a
dimensionless scalar quantity which is the ratio of
the force of friction, Ff between two bodies and
the normal reaction force, R.
The magnitude of the coefficient of friction
depends on the materials in contact: steel on ice
(in ice skating) has a low coefficient of friction;
rubber sole on the ground (running) has a high
coefficient of friction.
The greater the interaction between the
molecules of the interfacing surfaces, the greater
the size of the coefficient of friction.
Coefficients of friction range in value between
zero and one, but can sometimes be higher

Question 2

Describe friction

Answer 2

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

The value of the force of friction, Ff is calculated using: Ff = μR
Where μ is the coefficient of friction and R is the normal reaction force.

Question 3

Distinguish between the
coefficient of static friction
and dynamic friction.

Answer 3

When a force is applied to attempt to move a stationary object over another surface, we consider the coefficient of static friction. At some point, the force applied is sufficient to overcome
the static friction and the object will begin to move. Once the object is in motion, we consider the coefficient of dynamic friction. The coefficient
of dynamic friction is usually lower than the coefficient of static friction

Question 4

Explain the influence
of friction on sports
performance

Answer 4

Consider maximizing and minimizing frictional
influences in order to enhance performance.
For example:
* sports shoes (including spikes/cleats) and
playing surface (grass, artificial surfaces,
wood)
* winter sports (skiing, ice skating)
* use of a golf glove
* cycling on an indoor sloping velodrome

Advantages
Disadvantages
- good in sports such as sprinting because
prevents feet from slipping on the surface
- frictional forces help to move the object by friction (ex. spikes to run fast or studs in football)
- with friction, the ridges in our skin can helps us grab objects and hold on to them (ex. gripping a badminton/tennis racket)

Dis
- bad in sports such as skiing because you want minimal friction in order to slide and accelerate (waxing skis can help reduce friction)
- friction makes it more difficult when
one
has to move and
object (excess friction can
make it difficult to slide a box across the floor or walk through deep snow)
- excess friction =
extra energy, therefore
energy is being wasted

Question 5

Define drag

Answer 5

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

Question 6

Outline different types of
drag that can be found
in a variety of sporting
environments

Answer 6

Limit to:
* surface drag: As a body moves through a fluid, its outer surface catches a layer of the fluid nearby, slowing it down compared to the fluid further away and so causing drag. This can be minimized by changing the surface to reduce the interaction between surface and fluid. Example: The use of
shark-skin suits in swimming or shaving the swimmer’s body to make it smooth.
* form drag: As a body pushes against a fluid, the fluid pushes back (action and reaction). By streamlining the body and minimizing the surface area facing the direction of the motion, this type of drag is reduced.
Example: Cyclists adopting a low profile
position.
* wave drag: When a body moves along the
surface of a fluid (usually water) some fluid is displaced to form a wave. These waves cause additional forces that oppose motion. Wave drag can be reduced by avoiding motion at the interface between air and water.
Example: Swimming underwater for as long as is allowed at the start of a race

Question 7

Discuss factors that influence
the amount of drag in sports

Answer 7

Consider the influence of fluid viscosity, surface
size, shape, texture and relative velocity on drag.
For example:
* clothing for skiers, swimmers, runners,
cyclists and base jumpers
* equipment for cycling (helmet and bicycle
design)
* body position for a speed skater and
swimmer. Drag increases dramatically with
speed. (It increases as the square of the
speed.)
Aim 8: Consider the economic implications of
developing technologies to improve performance
in sports.
Int: Consider the availability of performanceenhancing technologies in different parts of the
world.

Question 8

Annotate a free-body
diagram showing the
direction of relevant forces
acting on an athlete or
object in sports.

Answer 8

Friction
Ground reaction force
Body weight
Air resistance
Push