Download 6.2 Friction

Friction
• Friction results from relative motion
between objects.
• Frictional forces are forces that resist or
oppose motion.
Types of Friction
• Static friction
• Sliding friction
• Rolling friction
Types of Friction
• Air friction
• Viscous friction
CAN I THINK ….?
Friction
Friction force (N)
Ff = μ. RN
Normal force (N)
Coefficient of friction
Calculate force of friction
• A 10 N force pushes down on a box that weighs 100 N.
• As the box is pushed horizontally, the coefficient of sliding friction is
0.25.
• Determine the force of friction resisting the motion. Also find F at
the point of sliding ?
Sliding Friction
Friction force (N)
Fs = msFn
Normal force (N)
Coefficient of
sliding friction
COMPARISON
•
•
•
•
STATIC
Ff OPPOSITE TO F
Ff = F
Ff < Fl
Ff / Rn = CONSTANT
= μ = tanφ
 Ff ≠ f ( AREA)
•
•
•
•
SLIDING
Fs OPPOSITE TO MOTION
F > Fl
Fs < Fl
Fs / Rn = CONSTANT
= μ = tanφ
• Ff ≠ f ( VELOCITY)
Table of friction coefficients
I THINK…. I CAN THINK….
Calculate using friction
• A steel pot with a weight of 50 N sits on a steel
countertop.
• How much force does it take to start the pot sliding?
Calculate using friction
• The engine applies a forward force of
1,000 Newton to a 500-kg car.
• Find the acceleration of the car if the
coefficient of rolling friction is 0.07.
LAWS OF FRICTION
• Basic properties of friction have been described as laws:
• Amontons' 1st Law: The force of friction is directly
proportional to the applied load.
• Amontons' 2nd Law: The force of friction is independent
of the apparent area of contact.
• Coulomb's Law of Friction: Kinetic friction is independent
of the sliding velocity.
• Amontons' 2nd Law is an idealization assuming perfectly
rigid and inelastic materials. For example, wider tires on
cars provide more traction than narrow tires for a given
vehicle mass because of surface deformation of the tyre.
ENERGY IN FRICTION
• According to the law of conservation of
energy, no energy is destroyed due to
friction, though it may be lost to the system
of concern. Energy is transformed from other
forms into heat
• When an object is pushed along a surface,
the energy converted to heat is given by:
• E = μ ∫ RN . x dx
WORK IN FRICTION
• In the reference frame of the interface between two
surfaces, static friction does no work, because there is
never displacement between the surfaces. In the
same reference frame, kinetic friction is always in the
direction opposite the motion, and does negative
work.
• The work done by friction can translate into
deformation, wear, and heat that can affect the
contact surface properties (even the coefficient of
friction between the surfaces). This can be beneficial
as in polishing. The work of friction is used to mix and
join materials such as in the process of friction
welding.
FRICTION AN ENGINEERING PROBLEM
• Bearing seizure or failure may result from
excessive wear due to work of friction. As
surfaces are worn by work due to friction, and
surface finish of an object may degrade until it
no longer functions properly.
APPLICATION
• SCREW JACK / LIFTING MACHINE
• INCLINED PLANE
• WEDGE
FRICTION ANGLE
• The angle of repose or, more precisely,
the critical angle of repose is the
steepest angle of descent or dip of the
slope relative to the horizontal plane
when material on the slope face is on
the verge of sliding. This angle is given
by the number (0°-90°) If the
coefficient of static friction is known of
a material, then a good approximation
of the angle of repose can be made
with the following function.
• μ = tan φ , Where:
μ is the coefficient of static friction
φ is the angle of repose
TYPICAL PROBLEMS….
WITH INCLINED PLANE
RN
F
W SIN θ
W COS θ
W
θ
CAN I INTROSPECT…….?
TYPICAL SITUATIONS
• NUMERICALS……..
• On to the white board………