Topic 10: Friction and Drag

• Friction


  a force that acts parallel to the interface of two surfaces that are in contact and opposes their relative motion
• Drag


  the force or forces acting to oppose the motion of an object through a fluid medium such as air or water
• 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 dragExample: high performance swimsuit material
• 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.