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1
Lecture 20
•
Statics
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The conditions for equilibrium are
0
0
=
=
∑
∑
τ
F
To have static equilibrium
p
= 0 and
L
= 0 too.
3
Center of gravity
If
g
is the same for all points within a body, then
r
cm
=
r
cg
.
An object that is 30 km tall will have
a 1% variation in
g
over its length.
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Consider a beam on a support that is beneath the beam’s
center of gravity.
The FBD for
the beam:
F
sb
F
eb
Apply N2L
0
eb
sb
=

=
∑
F
F
F
y
y
x
5
What about the torque?
0
2
2
eb
sb
=

=
∑
F
L
F
L
τ
If the axis is chosen at the point of contact between
the beam and support, the lever arm is zero for both
forces, and the net torque is zero.
This condition will
be met for any other axis as well.
Choose the axis at the left end of the beam:
This implies that F
sb
= F
eb
, the same as N2L.
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What if the support is moved a distance x to the left?
x
The FBD for
the beam:
F
sb
F
eb
Apply N2L
0
eb
sb
=

=
∑
F
F
F
y
y
x
7
What about the torque?
0
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This note was uploaded on 07/04/2011 for the course PHYS 121 taught by Professor Staff during the Spring '11 term at ASU.
 Spring '11
 Staff
 Gravity, Static Equilibrium

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