Transcript FRICTION

FRICTION
Section 2.7
Analyzing the Forces Acting on
the Shoe
 Constant Velocity
 occurs only when there is no net force on the shoe
 All the forces must add up to zero
 What was the force pulling back on the shoe when you
moved the shoe forward at a constant speed?
Vocabulary
 Friction – a force that resists relative motion between
two bodies in contact
Analyzing the Forces Acting on
the Shoe
 If gravity is pushing the shoe down, why is the shoe not
accelerating down?
Vocabulary
 Normal force – the force acting perpendicularly or
at right angles to a surface; “normal” means
“perpendicular to”
 Friction – a force that resists relative motion between
two bodies in contact
Vocabulary
 Coefficient of sliding friction (µ) – the ratio of
two forces; µ = force of friction (Ff)/perpendicular
force exerted by the surface on the object (normal
force) (FN); pronounced “mew”
 Normal force – the force acting perpendicularly or
at right angles to a surface; “normal” means
“perpendicular to”
 Friction – a force that resists relative motion between
two bodies in contact
Coefficient of Sliding Friction (µ)
 Noteworthy characteristics of µ
 It does not have a unit
 It usually is expressed in decimal form, such as 0.85
for rubber on dry concrete (0.60 on wet concrete)
 It is valid only for the pair of surfaces in contact
when the value is measured; any significant change in
either of the surfaces may cause the value of µ to
change
Practice!
 A vehicle having a mass of 1000 kg had an accident on a wet, but
level, concrete road under foggy conditions. The tires were
measured to have µ = 0.55 on wet concrete. The driver locked the
brakes, skidded for 6 seconds, and then hit the guardrail causing a
very small dent because the vehicle stopped just as it touched the
guardrail. The driver claimed to be driving 65 miles per hour (29
m/s).You have been hired as an investigator to determine if the
driver is telling the truth.
What Do You Think Now?
 Why do some sports require special shoes?