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 (solids)
b. Rolling (solids)
c. Fluid (liquids/gas)
Sliding Friction Examples
Rolling Friction Example
Fluid Friction Examples
SOLIDS
Sliding / Rolling Friction
Factors affecting sliding 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.
Examples:
i.
ii.
Leather soles on wood gym floor vs. Rubber soles on wood gym
floor.
Pulling a sled on snow vs. grass.
Factors affecting sliding friction
3. Friction depends upon the smoothness of the surfaces in
contact.
Example:
Rough vs. smooth wood.
Factors affecting sliding friction
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
Helpful uses of Sliding Friction
1.
2.
3.
4.
5.
6.
Striking a match
Tires on road
Nail or screw in wood
Opening a jar lid with your hand
Gripping a bat or racquet
Bow on violin strings (plays music)
Harmful uses of Sliding Friction
1.
2.
3.
4.
5.
6.
Moving furniture on carpet
Sliding on gym floor (floor burn)
Hand on shovel handle (blisters)
Valves or slides sticking on trumpet/trombone
Drawers hard to open
Ring stuck on finger
Ways to reduce sliding friction
1. Lubricants (oil, grease, graphite, etc.)
2. Smoother surfaces
3. Wheels and bearings
FLUID FRICTION
Liquid / Gas
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 fly higher)
3. Streamlining
Sloped windshield; pointed nose; headlight covers, recessed
door handles; smooth, curved surfaces
Ways to reduce fluid friction - Streamlining