Transcript bYTEBoss 08

ConcepTest Clicker
Questions
Chapter 8
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
Question 8.1a
Bonnie and Klyde I
Bonnie sits on the outer rim of a
merry-go-round, and Klyde sits
midway between the center and the
rim. The merry-go-round makes
one complete revolution every
2 seconds.
Klyde’s angular velocity is:
a) same as Bonnie’s
b) twice Bonnie’s
c) half of Bonnie’s
d) one-quarter of Bonnie’s
e) four times Bonnie’s
w
Klyde
Bonnie
Question 8.1a
Bonnie and Klyde I
Bonnie sits on the outer rim of a
merry-go-round, and Klyde sits
midway between the center and the
rim. The merry-go-round makes
one complete revolution every
2 seconds.
Klyde’s angular velocity is:
a) same as Bonnie’s
b) twice Bonnie’s
c) half of Bonnie’s
d) one-quarter of Bonnie’s
e) four times Bonnie’s
The angular velocity w of any point
w
on a solid object rotating about a
fixed axis is the same. Both Bonnie
and Klyde go around one revolution
(2p radians) every 2 seconds.
Klyde
Bonnie
Question 8.1b
Bonnie and Klyde II
Bonnie sits on the outer rim of a
merry-go-round, and Klyde sits
midway between the center and the
rim. The merry-go-round makes one
revolution every 2 seconds. Who has
the larger linear (tangential) velocity?
a) Klyde
b) Bonnie
c) both the same
d) linear velocity is zero
for both of them
w
Klyde
Bonnie
Question 8.1b
Bonnie and Klyde II
Bonnie sits on the outer rim of a
merry-go-round, and Klyde sits
midway between the center and the
rim. The merry-go-round makes one
revolution every 2 seconds. Who has
the larger linear (tangential) velocity?
a) Klyde
b) Bonnie
c) both the same
d) linear velocity is zero
for both of them
Their linear speeds v will be
w
different because v = r w and
Bonnie is located farther out
Klyde
(larger radius r) than Klyde.
1
VKlyde  VBonnie
2
Follow-up: Who has the larger centripetal acceleration?
Bonnie
Question 8.2
Truck Speedometer
Suppose that the speedometer of
a truck is set to read the linear
a) speedometer reads a higher
speed of the truck but uses a
speed than the true linear speed
device that actually measures the
b) speedometer reads a lower speed
angular speed of the tires. If
than the true linear speed
larger diameter tires are mounted
on the truck instead, how will that c) speedometer still reads the true
affect the speedometer reading as
linear speed
compared to the true linear speed
of the truck?
Question 8.2
Truck Speedometer
Suppose that the speedometer of
a truck is set to read the linear
a) speedometer reads a higher
speed of the truck but uses a
speed than the true linear speed
device that actually measures the
angular speed of the tires. If
b) speedometer reads a lower speed
larger diameter tires are mounted
than the true linear speed
on the truck instead, how will that
affect the speedometer reading as
c) speedometer still reads the true
compared to the true linear speed
linear speed
of the truck?
The linear speed is v = wR. So when the speedometer measures
the same angular speed w as before, the linear speed v is actually
higher, because the tire radius is larger than before.
Question 8.3a
Angular Displacement I
An object at rest begins to rotate with
a constant angular acceleration. If
this object rotates through an angle q
in the time t, through what angle did it
rotate in the time ½ t?
a) ½ q
b) ¼ q
c) ¾ q
d) 2 q
e) 4 q
Question 8.3a
Angular Displacement I
An object at rest begins to rotate with
a constant angular acceleration. If
this object rotates through an angle q
in the time t, through what angle did it
rotate in the time ½ t?
The angular displacement is q =
1
2
a) ½ q
b) ¼ q
c) ¾ q
d) 2 q
e) 4 q
at 2 (starting from rest), and
there is a quadratic dependence on time. Therefore, in half the
time, the object has rotated through one-quarter the angle.
Question 8.3b
Angular Displacement II
An object at rest begins to rotate
with a constant angular acceleration.
If this object has angular velocity w
at time t, what was its angular
velocity at the time ½ t?
a) ½ w
b) ¼ w
c) ¾ w
d) 2 w
e) 4 w
Question 8.3b
Angular Displacement II
An object at rest begins to rotate
with a constant angular acceleration.
If this object has angular velocity w
at time t, what was its angular
velocity at the time ½ t?
a) ½ w
b) ¼ w
c) ¾ w
d) 2 w
e) 4 w
The angular velocity is w = at (starting from rest), and there is a
linear dependence on time. Therefore, in half the time, the
object has accelerated up to only half the speed.
Question 8.4
You are using a wrench to
loosen a rusty nut. Which
Using a Wrench
a
b
arrangement will be the
most effective in loosening
the nut?
c
d
e) all are equally effective
Question 8.4
You are using a wrench to
loosen a rusty nut. Which
Using a Wrench
a
b
arrangement will be the
most effective in loosening
the nut?
Because the forces are all the
same, the only difference
is the lever arm. The
arrangement with the largest
lever arm (case #2) will
provide the largest torque.
c
d
e) all are equally effective
Follow-up: What is the difference between arrangement 1 and 4?
Question 8.5
Two Forces
Two forces produce the same
a) yes
torque. Does it follow that they
b) no
have the same magnitude?
c) depends
Question 8.5
Two Forces
Two forces produce the same
a) yes
torque. Does it follow that they
b) no
have the same magnitude?
c) depends
Because torque is the product of force times distance, two different
forces that act at different distances could still give the same torque.
Follow-up: If two torques are identical, does that mean their forces
are identical as well?
Question 8.6
Closing a Door
In which of the cases shown below
a) F1
is the torque provided by the
b) F3
applied force about the rotation
axis biggest? For all cases the
magnitude of the applied force is
the same.
c) F4
d) all of them
e) none of them
Question 8.6
Closing a Door
In which of the cases shown below
a) F1
is the torque provided by the
b) F3
applied force about the rotation
axis biggest? For all cases the
magnitude of the applied force is
the same.
c) F4
d) all of them
e) none of them
The torque is t = rFsin, and so
the force that is at 90° to the lever
arm is the one that will have the
largest torque. Clearly, to close
the door, you want to push
perpendicularly!!
Follow-up: How large would the force have to be for F4?
Question 8.7
When a tape is played on a cassette
deck, there is a tension in the tape
that applies a torque to the supply
reel. Assuming the tension remains
constant during playback, how does
this applied torque vary as the
supply reel becomes empty?
Cassette Player
a) torque increases
b) torque decreases
c) torque remains constant
Question 8.7
When a tape is played on a cassette
deck, there is a tension in the tape
that applies a torque to the supply
reel. Assuming the tension remains
constant during playback, how does
this applied torque vary as the
supply reel becomes empty?
Cassette Player
a) torque increases
b) torque decreases
c) torque remains constant
As the supply reel empties, the lever arm decreases because the
radius of the reel (with tape on it) is decreasing. Thus, as the
playback continues, the applied torque diminishes.
Question 8.8a
A force is applied to a dumbbell
for a certain period of time, first
as in (a) and then as in (b). In
which case does the dumbbell
acquire the greater
center-of-mass speed ?
Dumbbell I
a) case (a)
b) case (b)
c) no difference
d) it depends on the rotational
inertia of the dumbbell
Question 8.8a
A force is applied to a dumbbell
for a certain period of time, first
as in (a) and then as in (b). In
which case does the dumbbell
acquire the greater
center-of-mass speed ?
Because the same force acts for the
same time interval in both cases, the
change in momentum must be the
same, thus the CM velocity must be
the same.
Dumbbell I
a) case (a)
b) case (b)
c) no difference
d) it depends on the rotational
inertia of the dumbbell
Question 8.8b
A force is applied to a dumbbell
for a certain period of time, first
as in (a) and then as in (b). In
which case does the dumbbell
acquire the greater energy ?
Dumbbell II
a) case (a)
b) case (b)
c) no difference
d) it depends on the rotational
inertia of the dumbbell
Question 8.8b
A force is applied to a dumbbell
for a certain period of time, first
as in (a) and then as in (b). In
which case does the dumbbell
acquire the greater energy ?
Dumbbell II
a) case (a)
b) case (b)
c) no difference
d) it depends on the rotational
inertia of the dumbbell
If the CM velocities are the same, the
translational kinetic energies must
be the same. Because dumbbell (b)
is also rotating, it has rotational
kinetic energy in addition.
Question 8.9
Moment of Inertia
Two spheres have the same radius and
equal masses. One is made of solid
aluminum, and the other is made from a
hollow shell of gold.
Which one has the bigger moment of
inertia about an axis through its
center?
a) solid aluminum
b) hollow gold
c) same
hollow
solid
same mass & radius
Question 8.9
Moment of Inertia
Two spheres have the same radius and
equal masses. One is made of solid
aluminum, and the other is made from a
hollow shell of gold.
Which one has the bigger moment of
inertia about an axis through its
center?
Moment of inertia depends on
mass and distance from axis
squared. It is bigger for the
shell because its mass is
located farther from the center.
a) solid aluminum
b) hollow gold
c) same
hollow
solid
same mass & radius
Question 8.10
Figure Skater
A figure skater spins with her arms
a) the same
extended. When she pulls in her arms,
she reduces her rotational inertia and b) larger because she’s rotating
faster
spins faster so that her angular
momentum is conserved. Compared to c) smaller because her rotational
her initial rotational kinetic energy, her
inertia is smaller
rotational kinetic energy after she pulls
in her arms must be
Question 8.10
Figure Skater
A figure skater spins with her arms
a) the same
extended. When she pulls in her arms,
she reduces her rotational inertia
b) larger because she’s rotating
and spins faster so that her angular
faster
momentum is conserved. Compared
to her initial rotational kinetic energy, c) smaller because her rotational
inertia is smaller
her rotational kinetic energy after she
pulls in her arms must be:
1
1
KErot = 2 I w2 = 2 L w (used L = Iw ).
Because L is conserved, larger w
means larger KErot. The “extra”
energy comes from the work she
does on her arms.
Follow-up: Where does the extra energy come from?
Question 8.11
Two Disks
Two different spinning disks have
the same angular momentum, but
disk 1 has more kinetic energy than
disk 2.
a) disk 1
b) disk 2
c) not enough info
Which one has the bigger moment of
inertia?
Disk 1
Disk 2
Question 8.11
Two Disks
Two different spinning disks have
the same angular momentum, but
disk 1 has more kinetic energy than
disk 2.
a) disk 1
b) disk 2
c) not enough info
Which one has the bigger moment of
inertia?
1
KE = 2 I w2 = L2 / (2 I)
(used L = I w).
Because L is the same,
bigger I means smaller KE.
Disk 1
Disk 2
Question 8.12
Spinning Bicycle Wheel
You are holding a spinning bicycle
wheel while standing on a
stationary turntable. If you
suddenly flip the wheel over so
that it is spinning in the opposite
direction, the turntable will:
a) remain stationary
b) start to spin in the same
direction as before flipping
c) to spin in the same direction
as after flipping
Question 8.12
Spinning Bicycle Wheel
You are holding a spinning bicycle
wheel while standing on a
stationary turntable. If you
suddenly flip the wheel over so
that it is spinning in the opposite
direction, the turntable will:
The total angular momentum of the
system is L upward, and it is
conserved. So if the wheel has
−L downward, you and the table
must have +2L upward.
a) remain stationary
b) start to spin in the same
direction as before flipping
c) start to spin in the same
direction as after flipping
Question 8.13
Balancing Rod
A 1-kg ball is hung at the end of a rod
a) ¼ kg
1-m long. If the system balances at a
b) ½ kg
point on the rod 0.25 m from the end
c) 1 kg
holding the mass, what is the mass of
d) 2 kg
the rod?
e) 4 kg
1m
1kg
Question 8.13
Balancing Rod
A 1-kg ball is hung at the end of a rod
a) ¼ kg
1-m long. If the system balances at a
b) ½ kg
point on the rod 0.25 m from the end
c) 1 kg
holding the mass, what is the mass of
d) 2 kg
the rod?
e) 4 kg
The total torque about the pivot
must be zero !!
The CM of the
same distance
rod is at its center, 0.25 m to the
X
right of the pivot. Because this
must balance the ball, which is
the same distance to the left of
the pivot, the masses must be
the same !!
mROD = 1 kg
1 kg
CM of rod
Question 8.14
Mobile
a) 5 kg
A (static) mobile hangs as shown
below. The rods are massless and
b) 6 kg
have lengths as indicated. The mass
c) 7 kg
of the ball at the bottom right is 1 kg.
d) 8 kg
What is the total mass of the mobile?
e) 9 kg
?
1m
2m
?
1 kg
1m
3m
Question 8.14
Mobile
a) 5 kg
A (static) mobile hangs as shown
below. The rods are massless and
b) 6 kg
have lengths as indicated. The mass
c) 7 kg
of the ball at the bottom right is 1 kg.
d) 8 kg
What is the total mass of the mobile?
e) 9 kg
Use torques in two steps: (1)
find the big mass on the bottom
?
left (lower rod only), and (2) use
the entire lower rod assembly
(with two masses) to find the
mass on top right. Finally, add
up all the masses.
1m
2m
?
1 kg
1m
3m
Question 8.15a
Tipping Over I
a) all
A box is placed on a ramp in the
configurations shown below. Friction
prevents it from sliding. The center of
mass of the box is indicated by a blue dot
in each case. In which case(s) does the
box tip over?
1
b) 1 only
c) 2 only
d) 3 only
e) 2 and 3
2
3
Question 8.15a
Tipping Over I
A box is placed on a ramp in the
configurations shown below. Friction
prevents it from sliding. The center of
mass of the box is indicated by a blue dot
in each case. In which case(s) does the
box tip over?
a) all
b) 1 only
c) 2 only
d) 3 only
e) 2 and 3
The torque due to gravity acts
like all the mass of an object is
concentrated at the CM.
Consider the bottom right corner
of the box to be a pivot point.
If the box can rotate such that
the CM is lowered, it will!!
1
2
3
Question 8.15b Tipping Over II
Consider the two configurations of
a) case 1 will tip
books shown below. Which of the
b) case 2 will tip
following is true?
c) both will tip
d) neither will tip
1
2
1/4
1/2
1/2
1/4
Question 8.15b Tipping Over II
Consider the two configurations of
a) case 1 will tip
books shown below. Which of the
b) case 2 will tip
following is true?
c) both will tip
d) neither will tip
The CM of the system is
midway between the CM of
1
2
each book. Therefore, the
CM of case #1 is not over the
table, so it will tip.
1/4
1/2
1/2
1/4