Unit 10: Friction and drag Flashcards
How to calculate friction force?
Ff = μ · R
μ= coefficient of friction
R= normal reaction force
Describe friction
A force that opposes relative movement parallel to the interaction of two surfaces in contact.
Define the coefficient of friction
A dimensionless scalar quantity (no direction) which is the ratio of the force of friction, Ff and the normal reaction force, R:
μ=(Ff)/R
What does the magnitude of coefficient depend on?
- Materials in contact:
- steel on ice (in ice skating) has a low coefficient
- rubber sole on the ground (running) has a high
coefficient of friction.
- Range: 0-1
- Can be static or dynamic → dynamic is always lower than the one of static friction
- The greater the interaction between the molecules of the interfacing surfaces, the greater the size of the coefficient of friction.
Distinguish between the coefficient of static friction and dynamic friction
- Coefficient of static friction: A force is applied to attempt to move a stationary object over another surface
- Coefficient of dynamic friction: At some point, the force applied is sufficient to overcome the static friction and once the object is in motion
–> Coefficient of dynamic friction is lower than the coefficient of static friction.
Define drag:
Opposing force produced against the motion of an object moving through a fluid (air or water)
How is drag caused by air called?
Air resistance
Outline different types of drag that can be found in a variety of sporting environments
- Surface drag: refers to the interaction between body outer surface (material) and the fluid, which slows the body when moving and causing drag.
–> How can it be minimized? Changing the surface to reduce the interaction between surface and fluid.
–> EXAMPLE:
a. shaving (make smooth)
b. water-repellent (shark-skin) suit in swimming - Form drag: refers to the resistive forces caused by shape of object or body
–> How can it be minimized? By streamlining the body and minimizing the surface area facing the direction of the motion.
–> EXAMPLE: - Cyclists adopting a low profile position (crouch)
- Swimmers streamlining
- Cyclist’ helmet shape.
- Wave drag: opposing force caused by the body or object making waves in fluid as it moves along→ These waves cause additional forces that oppose motion.
–> How can it be reduced? Avoiding motion at the interface between air/water.
–> EXAMPLE: Swimming underwater for as long as allowed at the start of race.
Discuss factors that influence the amount of drag in sports:
- Fluid viscosity
- Surface size and shape
- Texture
- Relative velocity on drag
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