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Physics 1D03  Lecture 17
Statics (II)
Serway 12.3
•
More examples with friction
•
Internal forces in simple structures
Practice problems: Chapter 12, Problems 17, 51, 55, 57, 67.
Physics 1D03  Lecture 17
Example: rolling a spool.
inner radius a,
outer radius
b
= 2
a
μ
s
= 0.60 at both surfaces
If the spool has weight w = 100 N,
what minimum force
P
is needed to
rotate the spool?
b
a
C
B
A
P
Plan: Assume the spool is
not
rotating (
still in equilibrium
) but is
about to slip
(so
f
s
=
s
N
at
each
contact). Use the conditions for
equilibrium, and the two friction equations, to solve for
P
.
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Physics 1D03  Lecture 17
1) Freebody diagram:
b
a
C
B
A
b
= 2
a
w
= 100 N
μ
s
= 0.60
y
x
2) Equilibrium equations:
F
x
:
F
y
:
torques about C:
3) about to slip:
Physics 1D03  Lecture 17
b
a
C
B
A
b
= 2
a
w
= 100 N
s
= 0.60
QUIZ
What are the directions of the friction
forces at A and B?
P
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 Spring '10
 N.L.McKay
 Force, Friction, 3.0 m, µs, 1.2 m

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