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ConcepTest Clicker
Questions
Chapter 11
Physics, 4th Edition
James S. Walker
Copyright © 2010 Pearson Education, Inc.
Question 11.1
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 11.1
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 11.2
Two Forces
Two forces produce the same
a) yes
torque. Does it follow that they
b) no
have the same magnitude?
c) depends
Question 11.2
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 11.3
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 11.3
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 11.4
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 11.4
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 11.5a Dumbbell I
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 ?
a) case (a)
b) case (b)
c) no difference
d) it depends on the rotational
inertia of the dumbbell
Question 11.5a Dumbbell I
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.
a) case (a)
b) case (b)
c) no difference
d) it depends on the rotational
inertia of the dumbbell
Question 11.5b Dumbbell II
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 ?
a) case (a)
b) case (b)
c) no difference
d) it depends on the rotational
inertia of the dumbbell
Question 11.5b Dumbbell II
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 ?
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 11.6
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 11.6
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 11.7
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 11.7
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 2 = 2 L  (used L = I ).
Because L is conserved, larger 
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 11.8
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 11.8
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 2 = L2 / (2 I)
(used L = I ).
Because L is the same,
bigger I means smaller KE.
Disk 1
Disk 2
Question 11.9
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 11.9
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 11.10
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 11.10
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 11.11
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 11.11
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 11.12a
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 11.12a
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 11.12bTipping 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 11.12bTipping 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
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