Transcript Friction • Objectives

Friction
• Objectives
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Make simple measurements of distance and time
Learn graphing skills and understand graphical relationships
Understand the meaning and use of the slope in a motion diagrams.
Study friction and its affect on acceleration.
Understand Applications of Newton’s 2nd Law F = m a
Friction Forces
• Frictional forces oppose motion
– Usually proportional to the velocity
– Dependent upon the normal force FN
– Dependent upon the two surfaces through the coefficient of friction (m)
A: Max Normal Force, maximum friction
B: Normal Force with surface decreased, friction decreased
F
C: No Normal Force, No Friction
FN
B
F
A
f
F
C
Coefficient of Friction
• Depends upon the nature of the two surfaces
– Quantified by the coefficient of friction
m
• Does not depend upon the surface area or weight of
the sliding object, only on the nature of the surfaces.
m is LARGE for things like wood or sand paper
m is SMALL for things like plastic or teflon
Static and Kinetic Friction
Static Region
Kinetic Region
Ff max
Friction Force = Applied Force
Friction Force = mk FN
F
In the static region, the frictional force equals the applied force.
The frictional force depends upon the coefficient of static friction ms.
In the kinetic region, the object begins to move and the frictional force
decreases slightly to a constant value dependent upon the coefficient of
kinetic friction mk.
Smart Pulley System
The Smart Pulley allows us to make measurements of
displacement (Dd) and time (Dt). By plotting velocity (Dd/Dt)
of a laboratory cart, acceleration can be experimentally
determined as the slope in a motion diagram of velocity
versus time (Dv/Dt). m
T
c
T
mwg
Forces
Forces on block mass M (including friction)
Normal Force (FN = M g)
Gravity (weight)
Tension (from hanging mass weight)
Friction Ff = m FN
Forces on hanging mass (m)
Gravity (weight)
Tension
m a = F - Ff
(M + m) a = m g - m FN
m = {mg - (M+m)a}/Mg
g ~ 10 m/s2
Constant Acceleration?
• With constant non-zero acceleration…
 Velocity versus Time is linear.
 Velocity changes uniformly.
 Change in Velocity (Dv) over change in time (Dt) is
average acceleration.
• Average acceleration = Dv/Dt
Constant Acceleration Motion
Displacement
Time
Velocity
Slope = acceleration
Time