10 - Friction and drag

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

Friction = coefficient of friction x normal reaction force

• dimensionless scalar quantity What does ur show? - force of friction between two bodies and the normal reaction force What does it depend on? • the materials in contact with each other • the more interaction the greater size of the coefficient of friction • the value ranges from zero to 1 • greater interaction = greater coefficient of friction

A force is applied to attempt to move a stationary object over another surface.

At some point the force applied is sufficient to over come 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 lower than the coefficient of dynamic friction Dynamic friction > static friction

Bowling - shoes made of leather and rubber to help you slide, reducing friction. Avoids injury. 100 meter sprint - athletes have spikes on their shoes to increase the intersection to max their speed by maxing their coefficient of friction Ice skating - steel on ice has a low coefficient of friction Golf glove - tacky and helps to increase friction

A resistance to a movement that acts on any object through a fluid medium such as water

• Surface drag - a body moves through a fluid, it’s outer surface catches in the fluid slowly slowing it down How can it be reduced? - reduce interaction between surface and fluid (shaving legs) • Form drag - Body pushes against a fluid and the fluid will push back (action reaction) How can it be reduced? - being streamline, minimizing surface area (cyclists are streamline and crouched with helmets that are pointed) • Wave drag - when a body moves along a surface and the fluid displaces forming a wave, causing motion in the opposite direction How can it be reduced? - Swimming underwater for as long as possible

• Body position - Cyclists crouch • Equipment - helmets were first made of leather and not they are plastic and pointed • Clothing - used to be long and loose made from cotton and now they’re tight and short made from synthetic fibers that help remove sweat from skin, decreasing drag

A diagram used to show relative magnitude and direction of all the forces acting upon an object • size of the arrows reflect the magnitude of the force • direction of the arrow shows where it’s acting upon • each arrow is labeled • if arrows are balanced then the overall net force is 0 and the individual won’t move

• Air resistance (pushing towards a person) • body weight (down) • friction (from he behind to front) • push (front to back) • ground reaction force (pushing up)