Chapter Pretests


Chapter7
 
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Chapter7


Multiple Choice
Identify the choice that best completes the statement or answers the question.
 

 1. 
A uniform, horizontal beam of length 6.0 m and weight 120 N is attached at one end to a wall by a pin connection (so that it may rotate). A cable attached to the wall above the pin supports the opposite end. The cable makes an angle of 60° with the horizontal. What is the tension in the cable needed to maintain the beam in equilibrium?
a.
35 N
b.
69 N
c.
60 N
d.
120 N
 

 2. 
A 4.0-kg mass is placed at (3.0, 4.0) m, and a 6.0-kg mass is placed at (3.0, -4.0) m. What is the moment of inertia of this system of masses about the x-axis?
a.
160 kg×m2
b.
90 kg×m2
c.
250 kg×m2
d.
32 kg×m2
 

 3. 
The Earth moves about the Sun in an elliptical orbit. As the Earth moves closer to the Sun, which of the following best describes the Earth-Sun system's moment of inertia?
a.
decreases
b.
increases
c.
remains constant
d.
none of the above choices are valid
 

 4. 
A bucket of water with total mass 23 kg is attached to a rope, which in turn is wound around a 0.050-m 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 and the moment of inertia of cylinder and crank is 0.12 kg×m2. If the bucket is raised to the top of the well and released, what is the acceleration of the bucket as it falls toward the bottom of the well? (Assume rope's mass is negligible, that cylinder turns on frictionless bearings and that g = 9.8 m/s2.)
a.
3.2 m/s2
b.
6.3 m/s2
c.
7.4 m/s2
d.
9.8 m/s2
 

 5. 
A solid cylinder (I = MR2/2) has a string wrapped around it many times. When I release the cylinder, holding on to the string, the cylinder falls and spins as the string unwinds. What is the downward acceleration of the cylinder as it falls?
a.
0
b.
4.9 m/s2
c.
6.5 m/s2
d.
9.8 m/s2
 

 6. 
A gyroscope has a moment of inertia of 0.140 kg×m2 and has an initial angular speed of 15.0 rad/s. If a lubricant is applied to the bearings of the gyroscope so that frictional torque is reduced to 2.00 ´ 10-2 N×m, then in what time interval will the gyroscope coast from 15.0 rad/s to zero?
a.
150 s
b.
105 s
c.
90.0 s
d.
180 s
 

 7. 
A bus is designed to draw its power from a rotating flywheel that is brought up to its maximum speed (3 000 rpm) by an electric motor. The flywheel is a solid cylinder of mass 500 kg and radius 0.500 m (Icylinder = MR2/2). If the bus requires an average power of 10.0 kW, how long will the flywheel rotate?
a.
154 s
b.
308 s
c.
463 s
d.
617 s
 

 8. 
A ventilation fan with a moment of inertia of 0.034 kg×m2 has a net torque of 0.11 N×m applied to it. If it starts from rest, what angular momentum will it have 8.0 s later?
a.
0.88 kg×m2/s
b.
0.97 kg×m2/s
c.
2.0 kg×m2/s
d.
3.25 kg×m2/s
 

 9. 
The torque caused by a force F applied at a distance r from the axis of rotation, is given by mc009-1.jpg, where q is the angle between mc009-2.jpg and mc009-3.jpg. If the magnitudes r and F remain the same, what other angle q¢ will produce the same torque as was produced at angle q if q was less than 90°?
a.
mc009-4.jpg
b.
mc009-5.jpg
c.
mc009-6.jpg
d.
None of the above.
 

 10. 
A uniform meter stick balances on a fulcrum placed at the 40 cm mark when a weight W is placed at the 30 cm mark. What is the weight of the meter stick?
a.
W
b.
2W
c.
W/2
d.
0.4 W
 

 11. 
When a car is weighed, it is allowed to move over a scale which records a reading as the front wheels go over the scale, and a second reading when the rear wheels go over the scale. The weight of the car is equal to
a.
the weight under the front wheels.
b.
the weight under the rear wheels.
c.
the average of the two weights.
d.
the sum of the two weights.
e.
the difference of the two weights.
 

 12. 
As you are leaving a building, the door opens outward. If the hinges on the door are on your right, what is the direction of the angular velocity of the door as you open it?
a.
up
b.
down
c.
to your left
d.
to your right
e.
forwards
 

 13. 
When you ride a bicycle, in what direction is the angular velocity of the wheels?
a.
to your left
b.
to your right
c.
forwards
d.
backwards
e.
up
 

 14. 
A man is holding an 8.00-kg vacuum cleaner at arm's length, a distance of 0.550 m from his shoulder. What is the torque on the shoulder joint if the arm is held at 30.0° below the horizontal?
a.
21.6 Nm
b.
2.20 Nm
c.
4.40 Nm
d.
12.6 Nm
e.
37.4 Nm
 

 15. 
A 120-kg refrigerator, 2.00 m tall and 85.0 cm wide has its center of mass at its geometrical center. You are attempting to slide it along the floor by pushing horizontally on the side of the refrigerator. The coefficient of static friction between the floor and the refrigerator is 0.300. Depending on where you push, the refrigerator may start to tip over before it starts to slide along the floor. What is the highest distance above the floor that you can push the refrigerator so that it won't tip before it begins to slide?
a.
0.710 m
b.
1.00 m
c.
1.21 m
d.
1.42 m
e.
1.63 m
 

 16. 
A meter stick balances at the 50.0-cm mark. If a mass of 50.0 g is placed at the 90.0-cm mark, the stick balances at the 61.3-cm mark. What is the mass of the meter stick?
a.
127 g
b.
178 g
c.
89.7 g
d.
32.6 g
e.
73.4 g
 

 17. 
A stepladder consists of two halves, hinged at the top, and connected by a tie rod which keeps the two halves from spreading apart. In this particular instance, the two halves are 2.50 m long, the tie rod is connected to the center of each half and is 70.0 cm long. An 800-N person stands 3/5 of the way up the stepladder, as shown in Figure 11-8. Neglecting the weight of the ladder, and assuming that the ladder is resting on a smooth floor, what is the tension in the tie rod? Note: to solve this problem you must "cut" the ladder in half and consider the equilibrium of forces and torques acting on each half of the ladder.
a.
140 N
b.
240 N
c.
280 N
d.
360 N
e.
560 N
 

 18. 
A 3.50-g bullet, traveling at 250 m/s, hits a hinged, light board at right angles, at a distance of  55.0 cm from its hinges, and emerges on the other side of the board with a speed of 150 m/s. The board is initially stationary but is free to rotate about its hinges and has a moment of inertia of 0.275 kg·m2. What is the angular speed of the board after the bullet emerges?
a.
0.700 rad/s
b.
3.05 rad/s
c.
6.10 rad/s
d.
0.0212 rad/s
e.
22.6 rad/s
 

 19. 
An apparatus for measuring the mechanical equivalent of heat uses a crank to turn a small aluminum cylinder, which is being retarded by a belt. Friction with the belt heats up the cylinder, and the work done is then set equal to the heat gained by the cylinder. In one such experiment, the radius of the cylinder is 3.00 cm, the tension on one side of the belt is 50.0 N and on the other it is zero N. How much work is done turning the crank 212 times?
a.
2000 J
b.
4000 J
c.
19600 J
d.
637 J
e.
9800 J
 

 20. 
A chain, 10.0 m in length and with a mass of 3.00 kg/m but negligible thickness, is wrapped around a spool with a radius of 25.0 cm and a moment of inertia (not including the chain) of 0.360 kg·m2. With 1.00 m of chain hanging over the side, the spool is released so that it can turn freely about its axle. What is the angular speed of the spool at the moment that 5.00 m of chain are hanging over the side?
a.
8.90 rad/s
b.
17.8 rad/s
c.
19.4 rad/s
d.
22.1 rad/s
e.
44.3 rad/s
 

 21. 
2 600 rev/min is equivalent to which of the following?
a.
2600 rad/s
b.
43.3 rad/s
c.
273 rad/s
d.
60 rad/s
e.
56 rad/s
 

 22. 
What is the angular speed about the rotational axis of the Earth for a person standing on the surface?
a.
7.3 ´ 10-5 rad/s
b.
3.6 ´ 10-5 rad/s
c.
6.28 ´ 10-5 rad/s
d.
3.14 ´ 10-5 rad/s
e.
5.72 ´ 10-5 rad/s
 

 23. 
A fan blade, initially at rest, rotates with a constant acceleration of 0.025 rad/s2. What is its angular speed at the instant it goes through an angular displacement of 4.2 rad?
a.
0.025 rad/s
b.
0.11 rad/s
c.
0.46 rad/s
d.
1.2 rad/s
e.
1.4 rad/s
 

 24. 
Starting from rest, a wheel undergoes constant angular acceleration for a period of time T. At which of the following times does the average angular acceleration equal the instantaneous angular acceleration?
a.
0.50 T
b.
0.67 T
c.
0.71 T
d.
all of the above
e.
none of the above
 

 25. 
A ventilation fan has blades 0.25 m in radius rotating at 20 rpm. What is the tangential speed of each blade tip?
a.
0.02 m/s
b.
0.52 m/s
c.
5.0 m/s
d.
20 m/s
e.
23 m/s
 



 
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