# Friction and Newtonâ€™s Laws (Chapter 6) by umsymums38

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```									   Friction and Newton’s Laws
(Chapter 6)
Recall from last year that there are two
static       sliding
types of friction: _______ and _______.

NOTATION:
Fs    Static
___ = _______ Friction
Fk    Kinetic    Sliding
___ = ________ or ________ Friction
A force is applied to a block,
attempting to pull it to the left….
No Pull            Friction
No _________

The pull is balanced by a
F      Fs
static frictional force
_______________________, so
move
the block does not _______.

Fs also
As F is increased, __________
F          Fs
increases
__________....
… Until, the Fs reaches its
maximum
__________ possible value,
F              Fs,max
and the block is on the verge
of “breaking free”.
At the instant the block
F
Fk      “breaks away”, the friction
kinetic
becomes _________ friction,
less
which is _____ than static,
accelerates
and the block ____________.

F               If the pulling force is reduced
Fk      to equal the kinetic friction,
move with
the block will _____________
constant velocity
_____________________.

If the entire        Frictional Force

sequence is
graphed…
Coefficients of Friction, ms & mk
The coefficient of static friction, ms is defined
as:
ms 
(Dimensionless quantity &
F   s , max   determined experimentally to
1 or 2 decimal places only!)
N
The coefficient of kinetic friction, mk is defined
as:
mk     F          (Dimensionless quantity &
k         determined experimentally to
N   1 or 2 decimal places only!)

Solving for the frictional forces yields:

Fs ,max  m s N                  Fk          mk N
The AP Formula Sheet gives:             Ffric  mN
Example 1:                           P
A Pushing Force, P, is      q
applied to a block of
weight, W, at an angle, q,
causing the block to move at constant
velocity. Assume ms and mk are known.
Draw the FBD    Psinq      P
q
of the block:           Pcosq
accel  ____
0                              Fk

 F  ____
0                      W   N
(a) Find an expression for the pushing
force, P, in terms of W, q, and mk.
F    x
0         F    y
0
P cos q  Fk    N  W  P sin q
Fk  mk N
P cos q  mk W  P sin q 
Pcos q  mk sin q   mkW
m kW
P
cos q  m k sin q
(b) If ms is larger than some critical
value, the crate cannot be moved, no
matter how hard you push. Find this
maximum value of ms.
In this situation, again, the acceleration
and the net force are zero, but the
static
friction is now ________.
The expression for P is then…
m sW
P
cos q  m s sin q
The words “no matter how hard you
infinity
push” imply that P → __________, which
denominator
will happen when the ____________ of
0
the expression → ___.
cos q  m s sin q  0
m s  cos q sin q  1
tan q
Special Cases:
1 
• If q = 0°, ms = ____
0
1 0
• If q = 90° , ms = ____

• If q = 45° , ms = ____  1
1
tan 45

```
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