Topic 10 Flashcards
Describe friction
A force that acts parellel to the interface of two surfaces that are in contace, and opposes their relative motion
Ff= u x r
u: Coefficient of friction force
R: Normal reaction force
Describe the coefficient of friction
The coefficient of friction is a dimensionless scalar quantity which is the ration of the force of friction between two bodies and the normal reaction force
U: Ff/ R
The magnitude of the U depends on materials in contact:
- Steel on ice (iceskating) has low U
- Rubber sole on ground (running) has high U
The greater the interaction between the molecules of the interfacing surface, the greater the size of U
U can range between 0-1, but can sometimes be higher
Distinguish between the coefficient of static friction and dynamic friction
Static friction: When a force is applied to move a stationary object over another surface. At some point, the force applied is sufficient to overcome the static friction, and the object will begin to move
Dynamic friction: once the object is in motion. The coefficient of dynamic friction is usually lower than the coefficient of static friction.
Explain the influence of friction on sports preformance
- Sports shoes (cleats) and playing surface (grass, artifical surfaces, wood)
- Winter sports (skiing, iceskating)
- Golf gloves
- Cycling on an indoor velodrome
Define drag
Drag is the force or forces acting to oppose the motion of an object through a fluid medium such as air or water
Outline different types of drag that can be found in a variety of sporting environments
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
Ex: Shark-skin suits in swimming
sharing 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
Ex: 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
Ex: Swimming underwater for as long as is allowed at the start of a rac
Discuss factors that influence the amount of drag in sports
· fluid viscosity: Fluids resistance to flow’-> viscosity Fluid = ↑ resistance = ↑ drag
Surface size: ‘area of the objects surface that is exposed to the fluid flow’-> ↑ Surface area = ↑ drag
shape: smooth, streamlined shape ↓ drag Irregular, blunt =& drag
· texture: rough, textured = ↑ drag = ↑ friction
relative velocity on drag: velocity of object with respect to surrounding fluid. A faster = ↑ drag
Annotate a free-body diagram showing the direction of relevant forces acting on an athlete or object in sports
- Air resistance
- Body weight
- Push
- Friction
- Ground reaction force
