Download 4-8 friction - mrhsluniewskiscience

Objectives: The student will be able to:
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Distinguish between the different types of
friction.
Use free body diagrams and Newton's laws
of motion to solve word problems.
Calculate and compare the coefficients of
friction of various surfaces.
Phone Book and Friction
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
Separate the phone books
http://www.youtube.com/watch?v=hOt-D_eeJE
TWO types of Friction
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Static – Friction that keeps an object at rest
and prevents it from moving
Kinetic – Friction that acts during motion
Types of Friction
• Kinetic friction occurs when force is applied to an
object and the object moves.
• Examples:
Sliding Friction: pushing an object across a surface
Rolling Friction: between wheels and a surface
Fluid Friction: opposes the motion of objects
traveling through a fluid (air or water)
Affecting Friction
• To reduce the amount of friction, apply a lubricant
between two surfaces.
• Motor oil, wax, and grease are examples.
• Friction can also be reduced by rolling, rather than
pushing, an object.
Affecting Friction
• Friction increases as surfaces are made
rougher.
• Friction increases when the force between
two objects is increased.
Friction Forces
When two surfaces are in contact, friction forces
oppose relative motion or impending motion.
P
Friction forces are parallel to
the surfaces in contact and
oppose motion or impending
motion.
Static Friction: No
relative motion.
Kinetic Friction:
Relative motion.
Friction and the Normal Force
n
12 N
8N
n
4N
n2 N
4N
The force required to overcome static or
kinetic friction is proportional to the normal
force, n.
fs = msn
fk = mkn
6N
Friction forces are independent of area.
4N
4N
If the total mass pulled is constant, the same
force (4 N) is required to overcome friction
even with twice the area of contact.
For this to be true, it is essential that ALL
other variables be rigidly controlled.
Friction forces are independent of
temperature, provided no chemical or
structural variations occur.
4N
4N
Heat can sometimes cause surfaces to
become deformed or sticky. In such cases,
temperature can be a factor.
Friction forces are independent of speed.
5 m/s
20 m/s
2 N
2 N
The force of kinetic friction is the same at
5 m/s as it is for 20 m/s. Again, we must
assume that there are no chemical or
mechanical changes due to speed.
The Static Friction Force
When an attempt is made to move an object
on a surface, static friction slowly
increases to a MAXIMUM value.
n
P
fs
f s  ms
n
W
When we use the following equation, we
refer only to the maximum value of static
friction and simply write:
fs = msn
Constant or Impending Motion
For motion that is impending and for
motion at constant speed, the resultant
force is zero and SF = 0. (Equilibrium)
fs
P
Rest
P – fs = 0
fk
P
Constant Speed
P – fk = 0
Here the weight and normal forces are
balanced and do not affect motion.
Friction and Acceleration
When P is greater than the maximum fs the
resultant force produces acceleration.
a
fk
P
Constant Speed
fk = mkn
Note that the kinetic friction force remains
constant even as the velocity increases.
4-8 Applications Involving Friction, Inclines
On a microscopic scale, most
surfaces are rough. The exact
details are not yet known, but
the force can be modeled in a
simple way.
For kinetic – sliding –
friction, we write:
is the coefficient
of kinetic friction, and
is different for every
pair of surfaces.
4-8 Applications Involving Friction, Inclines
Friction
If the coefficient of kinetic friction between a 35-kg crate and the floor is
0.30, what horizontal force is required to move the crate to the right at
a constant speed across the floor?
Fa  Ff
Fn
Fa
Ff  mk FN
Fa  mk FN
FN  mg
Ff
Fa  mk mg
mg
Fa  (0.30)(35)(9.8)
Fa 
102.9 N
Friction
Suppose the same 35 kg crate was not moving at a constant speed, but
rather accelerating at 0.70 m/s/s. Calculate the applied force. The
coefficient of kinetic friction is still 0.30.
FNET  ma
Fa  Ff  ma
Fn
Fa
Fa  mk FN  ma
Fa  mk mg  ma
Fa  ma  mk mg
Ff
Fa  (35)(0.70)  (0.30)(35)(9.8)
mg
Fa 
127.4 N
4-8 Applications Involving Friction, Inclines
Static friction is the frictional force between two
surfaces that are not moving along each other.
Static friction keeps objects on inclines from
sliding, and keeps objects from moving when a
force is first applied.
4-8 Applications Involving Friction, Inclines
The static frictional force increases as the applied
force increases, until it reaches its maximum.
Then the object starts to move, and the kinetic
frictional force takes over.
Friction Lab
4-9 Problem Solving – A General Approach
1. Read the problem carefully; then read it again.
2. Draw a sketch, and then a free-body diagram.
3. Choose a convenient coordinate system.
4. List the known and unknown quantities; find
relationships between the knowns and the
unknowns.
5. Estimate the answer.
6. Solve the problem without putting in any numbers
(algebraically); once you are satisfied, put the
numbers in.
7. Keep track of dimensions.
8. Make sure your answer is reasonable.
Homework Chapter 4
Friction activity due in 2 days
Questions
#8, 12, 21
Problems
#s 36, 37
Homework Chapter 4 Problems
Friction activity due next class.
#s 38, 39, 44, 47