903
Chapter 12
Static Equilibrium and Elasticity
Conceptual Problems
1
•
(
a
) False. The conditions
∑
=
i
i
0
F
r
and
∑
=
i
i
0
τ
r
must be satisfied.
(
b
) True. The necessary and sufficient conditions for static equilibrium are
∑
=
i
i
0
F
r
and
∑
=
i
i
0
τ
r
.
(
c
) True. The conditions
∑
=
i
i
0
F
r
and
∑
=
i
i
0
τ
r
must be satisfied.
(
d
) False. An object is in equilibrium provided the conditions
∑
=
i
i
0
F
r
and
∑
=
i
i
0
τ
r
are
satisfied.
2
•
False. The location of the center of gravity depends on the mass distribution.
3
•
No. The definition of the center of gravity does not require that there be any material at its
location.
4
•
Determine the Concept
When the acceleration of gravity is not constant over an object,
the center of gravity is the pivot point for balance.
5
••
Determine the Concept
This technique works because the center of mass must be
directly under the balance point. Thus, a line drawn straight downward will pass through
the center of mass, and another line drawn straight downward when the figure is hanging
from another point will also pass through the center of mass. The center of mass is where
the lines cross.
*6
•
Determine the Concept
No. Because the floor can exert no horizontal force, neither can
the wall. Consequently, the friction force between the wall and the ladder is zero
regardless of the coefficient of friction between the wall and the ladder.

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