In sample problem 12.02, the vertical force acting on the beam by the hinge is equal to а. Mg b. Tr+ mg С. Tr d. To

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Sample Problem 12.02 Balancing a leaning boom
Figure 12-6a shows a safe (mass M = 430 kg) hanging by a
rope (negligible mass) from a boom (a = 1.9 m and b =
2.5 m) that consists of a uniform hinged beam (m = 85 kg)
and horizontal cable (negligible mass).
Figure 12-6 (a)
A heavy safe is
hung from a
boom consisting
Cable
of a horizontal
(a) What is the tension T, in the cable? In other words, what is
the magnitude of the force T on the beam from the cable?
steel cable and a
uniform beam.
Beam com
Rope
(b) A free-body
diagram for the
beam.
KEY IDEAS
The system here is the beam alone, and the forces on it are
shown in the free-body diagram of Fig. 12-6b. The force
from the cable is T. The gravitational force on the beam
acts at the beam's center of mass (at the beam's center) and
is represented by its equivalent mg. The vertical component
of the force on the beam from the hinge is F, and the hori-
zontal component of the force from the hinge is F. The
force from the rope supporting the safe is T. Because beam,
rope, and safe are stationary, the magnitude of T, is equal to
the weight of the safe: T, = Mg. We place the origin O of an
xy coordinate system at the hinge. Because the system is in
static equilibrium, the balancing equations apply to it.
Hinge
(a)
M
Beam
mg
Here is the
F,
wise choice of
Calculations: Let us start with Eq. 12-9 (Tnet.z = 0). Note
that we are asked for the magnitude of force T, and not of
forces F, and F, acting at the hinge, at point O. To eliminate
F, and F, from the torque calculation, we should calculate
torques about an axis that is perpendicular to the figure at
point O. Then F, and F will have moment arms of zero. The
lines of action for T, T, and mg are dashed in Fig. 12-6b.
The corresponding moment arms are a, b, and b/2.
Writing torques in the form of r Fand using our rule about
signs for torques, the balancing equation Tet.z = 0 becomes
rotation axis.
(b)
F
magnitude F of the net force. Because we know T, we apply
the force balancing equations to the beam.
Calculations: For the horizontal balance, we can rewrite
Fst = 0 as
F, – T = 0,
(12-20)
(a)(T.) – (b)(T,) – (¿b)(mg) = 0.
(12-19)
and so
F, = T = 6093 N.
Substituting Mg for T, and solving for T, we find that
For the vertical balance, we write Fety = 0 as
gb(M + m)
F, - mg – T, = 0.
T =
a
Substituting Mg for T, and solving for F, we find that
(9.8 m/s²)(2.5 m)(430 kg + 85/2 kg)
F, = (m + M)g = (85 kg + 430 kg)(9.8 m/s²)
1.9 m
= 6093 N = 6100 N.
(Answer)
= 5047 N.
From the Pythagorean theorem, we now have
(b) Find the magnitude F of the net force on the beam from
the hinge.
F = VF} + F2
= V(6093 N)? + (5047 N)² = 7900 N.
(Answer)
KEY IDEA
Now we want the horizontal component F, and vertical
component F, so that we can combine them to get the
Note that Fis substantially greater than either the combined
weights of the safe and the beam, 5000 N, or the tension in
the horizontal wire, 6100 N.
PLUS Additional examples, video, and practice available at WileyPLUS
Transcribed Image Text:Sample Problem 12.02 Balancing a leaning boom Figure 12-6a shows a safe (mass M = 430 kg) hanging by a rope (negligible mass) from a boom (a = 1.9 m and b = 2.5 m) that consists of a uniform hinged beam (m = 85 kg) and horizontal cable (negligible mass). Figure 12-6 (a) A heavy safe is hung from a boom consisting Cable of a horizontal (a) What is the tension T, in the cable? In other words, what is the magnitude of the force T on the beam from the cable? steel cable and a uniform beam. Beam com Rope (b) A free-body diagram for the beam. KEY IDEAS The system here is the beam alone, and the forces on it are shown in the free-body diagram of Fig. 12-6b. The force from the cable is T. The gravitational force on the beam acts at the beam's center of mass (at the beam's center) and is represented by its equivalent mg. The vertical component of the force on the beam from the hinge is F, and the hori- zontal component of the force from the hinge is F. The force from the rope supporting the safe is T. Because beam, rope, and safe are stationary, the magnitude of T, is equal to the weight of the safe: T, = Mg. We place the origin O of an xy coordinate system at the hinge. Because the system is in static equilibrium, the balancing equations apply to it. Hinge (a) M Beam mg Here is the F, wise choice of Calculations: Let us start with Eq. 12-9 (Tnet.z = 0). Note that we are asked for the magnitude of force T, and not of forces F, and F, acting at the hinge, at point O. To eliminate F, and F, from the torque calculation, we should calculate torques about an axis that is perpendicular to the figure at point O. Then F, and F will have moment arms of zero. The lines of action for T, T, and mg are dashed in Fig. 12-6b. The corresponding moment arms are a, b, and b/2. Writing torques in the form of r Fand using our rule about signs for torques, the balancing equation Tet.z = 0 becomes rotation axis. (b) F magnitude F of the net force. Because we know T, we apply the force balancing equations to the beam. Calculations: For the horizontal balance, we can rewrite Fst = 0 as F, – T = 0, (12-20) (a)(T.) – (b)(T,) – (¿b)(mg) = 0. (12-19) and so F, = T = 6093 N. Substituting Mg for T, and solving for T, we find that For the vertical balance, we write Fety = 0 as gb(M + m) F, - mg – T, = 0. T = a Substituting Mg for T, and solving for F, we find that (9.8 m/s²)(2.5 m)(430 kg + 85/2 kg) F, = (m + M)g = (85 kg + 430 kg)(9.8 m/s²) 1.9 m = 6093 N = 6100 N. (Answer) = 5047 N. From the Pythagorean theorem, we now have (b) Find the magnitude F of the net force on the beam from the hinge. F = VF} + F2 = V(6093 N)? + (5047 N)² = 7900 N. (Answer) KEY IDEA Now we want the horizontal component F, and vertical component F, so that we can combine them to get the Note that Fis substantially greater than either the combined weights of the safe and the beam, 5000 N, or the tension in the horizontal wire, 6100 N. PLUS Additional examples, video, and practice available at WileyPLUS
In sample problem 12.02, the vertical force
acting on the beam by the hinge is equal to
а. Mg
b. Tr+ mg
С.
Tr
d. Tc
Transcribed Image Text:In sample problem 12.02, the vertical force acting on the beam by the hinge is equal to а. Mg b. Tr+ mg С. Tr d. Tc
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