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?