Download Unit 4.3 Friction Objectives Friction

Unit 4.3 Friction
Teacher: Dr. Van Der Sluys
Objectives
• Types of friction
– Static
– Kinetic
• Force of friction and the normal force
• Horizontal friction problems
• Incline problems with friction
Friction
• Friction is due to contact forces between two
surfaces.
• The force of friction (Ff) is perpendicular to the
normal force acting between two materials and
opposes the applied force (F a ) that causes sliding.
FN
Fa
Ff
surface
F
g
surface
1
Static Friction
• Static friction refers to a force parallel to the two
surfaces that can arise when the two materials are
not sliding across one another.
• On the molecular level, the ridges and valleys of the
two surfaces are locked together and the force of
static friction must overcome this resistance.
Surface 2
Surface 1
Kinetic Friction
• Kinetic friction is the resistance to motion that
results from two surfaces that interact while
skidding across one another while in contact.
Constant Velocity
Surface 2
Surface 1
The Friction Equation
FF = µ FN
FF = Force of friction
FN = Normal Force
µ = coefficient of friction
2
Coefficients of Friction
Surfaces
Coefficients of
Static Friction
(ms)
Coefficients of
Kinetic Friction
(mk)
Wood on Wood
0.4
0.2
Rubber on dry
concrete
1.0
0.8
Rubber on wet
concrete
0.7
0.5
Lubricated ball
bearings
<0.01
<0.01
Synovial Joints
(Humans)
0.01
0.01
Getting an Object to Move
• If an object is at rest, the applied horizontal
force must exceed the force of static friction.
• Once the object begins to slide, if the applied
force equals the force of kinetic friction the
object will move with constant velocity (no
acceleration).
• Once the object is moving, if the applied
force is greater than the force of kinetic
friction, the object will accelerate.
Example
• A 10.0 kg box rests on a horizontal
floor. The coefficient of static friction is
µs = 0.40 and the coefficient of kinetic
friction is µk = 0.30. Determine the
force of friction, FF, acting on the box if
a horizontal external applied force is
exerted with a magnitude of (a) 0 N; (b)
10 N; (c) 38 N; (d) 40 N.
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Pushing or Pulling a Sled
• If a child on a sled is on flat ground, will
you exert less force if you push down
on the back of the child or pull up on
the rope with the same angle.
• Draw the free-body diagrams.
Objects Sliding on an Incline
y
FN
Motion
FF
x
Fg
θ
Components of Weight
http://rt210.sl.psu.edu/phys_anim/mech/indexer_mech.html
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Example
• A 10.0 kg box is sliding down a 20°
incline with a constant velocity. If the
coefficient of kinetic friction is 0.50,
what is the force of friction that the
surface exerts on the box?
Answer 0.36
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