Friction and Newton’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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