Chapter 8, Rotational Equilibrium and Rotational Dynamics
103
C
HAPTER
8
8.1 Torque
1. A vault is opened by applying a force of 300 N perpendicular to the plane of the door, 0.80 m
from the hinges. Find the torque due to this force about an axis through the hinges.
a. 120 N
⋅
m
b. 240 N
⋅
m
c. 300 N
⋅
m
d. 360 N
⋅
m
2. A 3.0m rod is pivoted about its left end. A force of 6.0 N is applied perpendicular to the rod
at a distance of 1.2 m from the pivot causing a ccw torque, and a force of 5.2 N is applied at
the end of the rod 3.0 m from the pivot. The 5.2 N is at an angle of 30
o
to the rod and causes a
cw torque. What is the net torque about the pivot?
a. 15 N·m
b. 0 N·m
c. –6.3 N·m
d. –0.6 N·m
3. A rod of length L is pivoted about its left end and has a force F applied perpendicular to the
other end. The force F is now removed and another force F’ is applied at the midpoint of the
rod. If F’ is at an angle of 30
o
with respect to the rod, what is its magnitude if the resulting
torque is the same as when F was applied?
a. F
b. 2F
c. 3F
d. 4F
4.
Two children seat themselves on a seesaw. The one on the left has a weight of 400 N while the
one on the right weighs 300 N. The fulcrum is at the midpoint of the seesaw. If the child on the
left is not at the end but is 1.50 m from the fulcrum and the seesaw is balanced, what is the
torque provided by the weight of the child on the right?
a. 600 N·m
b. 450 N·m
c. –600 N·m
d. –450 N·m
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104
5. A bucket filled with water has a mass of 23 kg and is attached to a rope, which in turn, is
wound around a 0.050m radius cylinder at the top of a well. What torque does the weight of
water and bucket produce on the cylinder if the cylinder is not permitted to rotate? (
g
= 9.8
m/s
2
)
a. 34 N
⋅
m
b. 17 N
⋅
m
c. 11 N
⋅
m
d. 23 N
⋅
m
6. A bucket of water with total mass 23 kg is attached to a rope, which in turn, is wound around
a 0.050m radius cylinder at the top of a well. A crank with a turning radius of 0.25 m is
attached to the end of the cylinder. What minimum force directed perpendicular to the crank
handle is required to just raise the bucket? (Assume the rope’s mass is negligible, that cylinder
turns on frictionless bearings, and that
g
= 9.8 m/s
2
.)
a. 45 N
b. 68 N
c. 90 N
d. 135 N
8.2 Torque and the Two Conditions for Equilibrium
8.3 The Center of Gravity
8.4 Examples of Objects in Equilibrium
7. A uniform bridge span weighs 50.0
×
10
3
N and is 40.0 m long. An automobile weighing 15.0
×
10
3
N is parked with its center of gravity located 12.0 m from the right pier. What upward
support force does the left pier provide?
a. 29.5
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 isac
 Physics, Force, Kinetic Energy, Moment Of Inertia, A., C C B C C D B B B C D B A D C D B, D B C D C D B C C D A B C B B C

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