# 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
• 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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