Friction:

The Force that resists the movement of objects that are in contact with each other (FF). This resistive force is caused by the surface roughness of the contact area of the materials, molecular attraction or adhesion between materials, and deformations in the materials.

Causes of Friction:

If two surfaces are highly polished, the friction actually

increases. This resistive force is due to the attraction of

molecules of the substances.

Causes of Friction:

Most friction results because the surfaces of materials

being rubbed together are not completely smooth (Surface

irregularities).

If you looked at what seems to be a smooth surface under

a high-powered microscope, you would see bumps, hills

and valleys that could interfere with sliding motion.

Kinds of Friction

Static Friction (Ffs)

A force that opposes the motion of a stationary object.

The static friction force must be overcome by an applied force

before an object can move.

For example, static friction can prevent an object from sliding

down a sloped surface

Kinds of Friction

Kinetic Friction (Ffk)

A force that opposes the motion of a moving object.

Kinetic friction occurs when two objects are moving relative to

each other and rub together (like a sled on the ground)

a. Sliding (solid)

b. Rolling (solid)

c. Fluid (liquid/gas)

Sliding Friction

Rolling Friction

Fluid Friction

SOLIDS

Sliding / Rolling Friction

1. Friction acts parallel to the surfaces in contact and in the

opposite direction to the motion.

Factors affecting sliding friction

2. Friction depends upon the nature (kind) of the materials in

contact.

Example:

i. Leather soles on wood gym floor vs. Rubber soles on

wood gym floor.

ii. Pulling a sled on snow vs. grass.

Factors affecting sliding friction

3. Friction depends upon the smoothness of the surfaces

in contact.

Example: Rough (unplanned wood) vs. smooth (planed)

wood.

4. Friction is directly proportional to the force pressing the

surfaces together (FN). Therefore, as FN increases, Ff

increases.

Factors affecting sliding friction

5. Friction is independent of the area in contact.

Example: Tires

Wide treads, regular treads, narrow treads, bike tires

Factors affecting sliding friction

Helpful uses of Sliding Friction

1. Striking a match

2. Tires on road

3. Nail or screw in wood

4. Opening a jar lid with your hand

5. Brake pad on a disc brake

6. Gripping a bat or racquet

7. Bow on violin strings (plays music)

Harmful uses of Sliding Friction

1. Moving furniture on carpet

2. Pistons on cylinder walls

3. Sliding on gym floor (floor burn)

4. Hand on shovel handle (blisters)

5. Valves or slides sticking on trumpet/trombone

6. Drawers hard to open

7. Ring stuck on finger

Ways to reduce sliding friction

1. Lubricants (oil, grease, graphite, etc.)

2. Smoother surfaces

3. Wheels and bearings

http://en.wikipedia.org/wiki/File:BallBearing.gif

LIQUID / GAS

Fluid Friction

Factors affecting fluid friction

1. Fluid Friction increases as the speed of the body

moving through a fluid increases.

2. Fluid friction is greater for objects with greater surface

area.

3. Fluid friction depends upon the shape of the object

Example: The larger the parachute, the greater the air

friction.

Helpful uses of fluid friction

1. Slowing down the space shuttle & parachute for landing

2. Burns more calories during water aerobics

3. Burns up meteors

Harmful uses of fluid friction

1. Slowing down skis & boats in water

2. Oil flowing in a pipeline

3. Slowing down cars and planes

Ways to reduce fluid friction

1. Smooth surfaces (polish/wax)

2. Reduce viscosity of the fluid (Planes flying higher)

3. Streamlining

Sloped windshield; pointed nose; headlight covers,

recessed door handles; smooth, curved surfaces

Ways to reduce fluid friction - Streamlining