Unit 10: Friction and drag

Question 1

How to calculate friction force?

Answer 1

Ff = μ · R
μ= coefficient of friction
R= normal reaction force

Question 2

Describe friction

Answer 2

A force that opposes relative movement parallel to the interaction of two surfaces in contact.

Question 3

Define the coefficient of friction

Answer 3

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

Question 4

What does the magnitude of coefficient depend on?

Answer 4

1. 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.
2. Range: 0-1
3. Can be static or dynamic → dynamic is always lower than the one of static friction
4. The greater the interaction between the molecules of the interfacing surfaces, the greater the size of the coefficient of friction.

Question 5

Distinguish between the coefficient of static friction and dynamic friction

Answer 5

• 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.

Question 6

Define drag:

Answer 6

Opposing force produced against the motion of an object moving through a fluid (air or water)

Question 7

How is drag caused by air called?

Answer 7

Air resistance

Question 8

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

Answer 8

1. 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
2. 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:
3. Cyclists adopting a low profile position (crouch)
4. Swimmers streamlining
5. Cyclist’ helmet shape.
6. 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.

Question 9

Discuss factors that influence the amount of drag in sports:

Answer 9

1. Fluid viscosity
2. Surface size and shape
3. Texture
4. Relative velocity on drag