Download Forces of Friction

Forces of Friction
Contact between bodies with a relative velocity
produces friction
• Friction is proportional to the normal force
• The force of static friction is generally greater
than the force of kinetic friction
• The coefficient of friction (µ “mu”) depends on
the surfaces in contact
• The direction of the frictional force is opposite
the direction of motion
• The coefficients of friction are nearly
independent of the area of contact
Fig. 4-19, p.101
Static Friction ƒs
• Static friction acts to keep the object
from moving
• If F increases, so does ƒs
• If F decreases, so does ƒs
• ƒs ≤ µ n where n = normal force vector
uur uur
f s = -F
Kinetic Friction, ƒk
• The force of kinetic friction
acts when the object is in
motion
• ƒk = µ k n
– Direction of ƒk opposite to
motion: opposes motion
– Variations of coefficient
of friction with speed will
be ignored
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It is STRONGLY recommended to login your remote for every class just
to be sure it is on the correct radio channel and working before class.
1.
2.
3.
4.
PRESS AND HOLD THE DOWN ARROW KEY until the
GREEN light on the remote turns RED.
PRESS THE “0” KEY and you will see the RED light flash
GREEN.
PRESS THE “7” KEY and you will see the RED light flash
GREEN.
PRESS AND RELEASE THE DOWN ARROW KEY again
and you will see the red light search for the receiver, if it
BLINKS GREEN MULTIPLE TIMES you are logged in.
In-class Quiz 9/21
If you press a book flat against a vertical wall with
your hand, in what direction is the friction force
exerted by the wall on the book?
1.
2.
3.
4.
5.
downward
upward
out of the wall
into the wall
Can not be
determined
In-class Quiz 9/21
A crate is sitting on the center of a flatbed truck.
As the truck accelerates to the east, the crate moves
with it, not sliding on the bed of the truck. In what
direction is the friction force exerted by the bed of
the truck on the crate?
1.
2.
3.
4.
5.
To the west
Upward
To the east
Downward
Can not be
determined
Ex. Suppose a block with a mass of 2.50
kg is resting on a ramp. If the coefficient of
static friction between the block and ramp is
0.35, what maximum angle can the ramp
make with the horizontal before the block
starts to slip down?
In-class Quiz 9/21
For the block on the ramp
the frictional force is pointed--
1.
2.
3.
4.
5.
Normal to the ramp
Down the down
Up the ramp
Out of the page
Can not be
determined
Block on a Ramp
• Axes are rotated as
usual on an incline
• The direction of
impending motion
would be down the
plane
• Friction acts up the
plane
– Opposes the motion
• Draw Free Body
Diagram yada yada
yada
Ex 4.13
m1=4kg
m2=7kg
µk =0.3
Find acc. and
tension.
Chapter 5.1-5.2
Energy and Work
•Mechanical Energy
•Kinetic (associated with motion)
•Potential (associated with position)
•Chemical Energy
•Electromagnetic Energy
•Nuclear Energy
Energy can be transformed from one form to another
But not destroyed--Conserved
Work/Energy can be used in place of Newton’s laws
to solve certain problems more simply
Work
W ≡ (F cos θ)∆x
F is the magnitude of the net force
∆ x is the magnitude of the
object’s displacement
r
r
θ is the angle between F and ∆x
• This gives no information about
– the time it took for the
displacement to occur
– the velocity or acceleration
of the object
• Work is a scalar quantity
Lecture Quiz 9/21
What are the units of Energy & Work?
A.
B.
C.
D.
E.
Newton • meter
kg • m2 / s2
foot • pound
Joule
All of the above
Units of Work
• SI
– Newton • meter = Joule
•N•m=J
• J = kg • m2 / s2
• US Customary
– foot • pound
• ft • lb
–no special name
Energy: ability to do work
Work is Zero when force and
displacement are perpendicular
Carrying a bucket of water
• Displacement is horizontal
• Force is vertical
• cos 90° = 0
So no work done!
Work Can Be Positive or Negative
• Work done on box is
positive when lifting the box
• Work is negative if lowering
the box
– The force would still be
upward, but the
displacement would be
downward
OR
Box does positive work on
student when lowered
Kinetic Energy
• Energy associated with the motion of an
object
1
KE = mv 2
•
2
• Scalar quantity with the same units as work
• Work is related to kinetic energy
Work-Kinetic Energy Theorem
• When work is done by a net force on an object and
the only change in the object is its speed, the work
done is equal to the change in the object’s kinetic
energy
Wnet = KEf − KEi = ∆KE
•
– Speed will increase if work is positive
– Speed will decrease if work is negative
Work and Kinetic Energy
• An object’s kinetic energy
can also be thought of as
the amount of work the
moving object could do in
coming to rest
– The moving hammer has
kinetic energy and can do
work on the nail