Slide 1 - AHS Physics by wpnt1Y7S

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									SCIENCE
FRICTION
“Hard to live with, can’t live without”
A
• discover and describe factors affecting
  friction
• gather and analyze data for verifying
  assumptions about friction
 ASSUMPTIONS
• Factors affecting friction:
  A. Weight
  B. Type of material
  C. Surface Area
           EXPERIMENT A
• Weight and Friction

 Analysis: Weight influences
 friction. The greater is the
 weight, the higher is the
 friction force.


       Trial         Weight     Friction
         1           4.45 N      2.00 N
         2           6.41 N      3.20 N
         3           8.42 N      4.50 N
             EXPERIMENT B
• Surfaces in contact and Friction

 Analysis: Type of material
 influences friction. Rubber
 on vinyl has greater friction
 than wood on vinyl.


     Trial          Surfaces         Friction
       1         Vinyl on wood        1.90 N
       2         Vinyl on rubber      2.20 N
             EXPERIMENT C
• Size of Contact Area and Friction

 Analysis: Surface area does
 not influence friction. There
 is no difference in the
 magnitude of friction force.


     Trial          Surfaces          Friction
       1              large            2.00 N
       2             small             2.00 N
CONCLUSIONS

• Factors affecting friction:
  A. Weight
  B. Type of material
• Independent of:
  A. Surface area
  B. Speed
      LAWS OF FRICTION
Law 1: Friction is parallel to the surface of
contact but opposite the direction of the
object’s motion
                             FN

                        FA
     Ff



                             Fg
         LAWS OF FRICTION
 Law 2: If the applied force does not cause
 motion, it is STATIC FRICTION. Likewise if the
 applied force causes motion, it is KINETIC
 FRICTION.


FA=50N   FA=100N   FA=150N    FA=200N    FA=250N




Ff=50N   Ff=100N    Ff=150N    Ff=125N    Ff=125N
       LAWS OF FRICTION
Law 3: The ratio of friction to the normal force is
equal to coefficient of friction, “mu” (µ)

                   µ = Ff / FN
      Materials                µs                 µk
rubber on dry concrete        0.80               0.65
rubber on wet concrete        0.60               0.40
    wood on wood              0.50               0.20
  steel on steel (dry)        0.78               0.58
  steel on steel (wet)        0.15               0.06
                           static friction   kinetic friction
PHYSICS
BUSTER!



   A girl exerts a 36-N horizontal
   force as she pulls a 52-N sled
   across a cement sidewalk at a
    constant speed. What is the
     coefficient of kinetic friction
   between the sidewalk and the
                 sled?
PHYSICS
BUSTER!



   A small child is dragging a heavy
    rubber soled shoe by its laces
   across a sidewalk at a constant
  speed of 0.35 m/s. If the shoe has
    a mass of 1.56 kg, what is the
  horizontal component of the force
        exerted by the child?
PHYSICS
BUSTER!



   You push a 25.0 kg wooden box
     across a wooden floor at a
   constant speed of 1.0 m/s. How
   much force do you exert on the
     box? If the force applied is
    doubled, what is the resulting
            acceleration?
PHYSICS
BUSTER!
              1. You need to move a 105 kg sofa to a different
              location in the room. It takes a force of 102 N to
              start it moving. What is the coefficient of static
              friction between the sofa and the carpet?


2. Ms. Anne Liwanag is dragging a box full of
books from her office to her car. The box and
books together have a combined weight of 134
N. If the coefficient of static friction between the
pavement and the box is 0.55, how hard must
Ms. Liwanag push the box in order to start it
moving?

								
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