25. Two bodies, A and B, have equal kinetic energies. The mass of A is nine times that of B.
The
ratio of the momentum of A to that of B is:
A. 1:9
B. 1:3
C. 1:1
D. 3:1
E. 9:1
ans: D

49. Two separated sources emit sinusoidal traveling waves that have the same wavelength

λ

and
are in phase at their respective sources. One travels a distance

�

1

to get to the observation

point while the other travels a distance

�

2

. The amplitude is a minimum at the observation

point if

�

1

−

�

2

is:

A. an odd multiple of

λ/2

B. an odd multiple of

λ/4

C. a multiple of

λ

D. an odd multiple of

π/2

E. a multiple of

π

ans: A

16. Nine identical wires, each of diameter d and length L, are connected in parallel. The
combination
has the same resistance as a single similar wire of length L but whose diameter is:
A. 3d
B. 9d
C. d/3
D. d/9
E. d/81
ans: A
17. Nine identical wires, each of diameter d and length L, are connected in series. The
combination
has the same resistance as a single similar wire of length L but whose diameter is:
A. 3d
B. 9d
C. d/3
D. d/9
E. d/81
ans: C

40. A freely falling body has a constant acceleration of 9.8 m/s

2

. This means that:

A. the body falls 9.8 m during each second
B. the body falls 9.8 m during the first second only
C. the speed of the body increases by 9.8 m/s during each second
D. the acceleration of the body increases by 9.8 m/s

2

during each second

E. the acceleration of the body decreases by 9.8 m/s

2

during each second

ans: C

37. The system shown remains at rest. Each block weighs 20 N. The force of friction on the
upper
block is:

..............................................................................................................................................................................................................................................................................................
.................................................................................................................................................................................................................................................................................................
................................................................................................................. ... ... .. ...........................................................
...................................................................................................................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................
.........................

.........................

.

a
b
W
W
W = 20N
a = 3m
b = 4m
A. 4N
B. 8N
C. 12N
D. 16N
E. 20N
ans: B

44. Why do raindrops fall with constant speed during the later stages of their descent?
A. The gravitational force is the same for all drops
B. Air resistance just balances the force of gravity
C. The drops all fall from the same height
D. The force of gravity is negligible for objects as small as raindrops
E. Gravity cannot increase the speed of a falling object to more than 9.8m/s
ans: B

59. An automobile moves on a level horizontal road in a circle of radius 30 m. The coefficient
of
friction between tires and road is 0.50. The maximum speed with which this car can round
this curve is:
A. 3.0m/s
B. 4.9m/s
C. 9.8m/s
D. 12m/s
E. 13m/s
ans: D

50. Two bodies of unequal mass, placed at rest on a frictionless surface, are acted on by
equal

horizontal forces for equal times. Just after these forces are removed, the body of greater
mass
will have:
A. the greater speed
B. the greater acceleration
C. the smaller momentum
D. the greater momentum
E. the same momentum as the other body
ans: E

69. A 16-kg block is attached to a cord that is wrapped around the rim of a flywheel of
diameter
0.40m and hangs vertically, as shown. The rotational inertia of the flywheel is 0.50 kg · m

2

.

When the block is released and the cord unwinds, the acceleration of the block is:

....................................................................................................... ........ ... ... ... ... ......... ....
......................................................................................................................................................................................................................................................................................................................................

.................

•

......................................................................................................................................................................................................................................................................................................................................
........................... ........... . ..........
.....................
.....................
.....................
.....................
.....................
.....................
.....................
.....................
.....................
.....................
.....................
.....................
.....................
.....................

16 kg
↑||
0.4m
||↓
A. 0.15g
B. 0.56g
C. 0.84g
D. g
E. 1.3g
ans: B

34. Two protons (p

1

and p

2

) and an electron (e) lie on a straight line, as shown. The

directions of
the force of p

1

on e, the force of p

2

on e, and the total force on e, respectively, are:

p•

1

•e p•

2

A. −→, ←−, −→ B. ←−, −→, −→ C. −→, ←−, ←− D. ←−, −→, ←− E. ←−, ←−,

17. Two protons (p

1

and p

2

) are on the x axis, as shown below. The directions of the electric

field
at points 1, 2, and 3, respectively, are:
p•

1

p•

2

× × ×

1 2 3
A. −→, ←−, −→ B. ←−, −→, ←− C. ←−, −→, −→ D. ←−, ←−, ←− E. ←−, ←−, −→
ans: E

20. An electron is launched with velocity

�v in a uniform magnetic field � B. The angle θ

between
�v and � B is between 0 and 90

◦

. As a result, the electron follows a helix, its velocity vector

�v
returning to its initial value in a time interval of:
A. 2

πm/eB

B. 2

πmv/eB

C. 2

πmv sin θ/eB

D. 2

πmv cos θ/eB

E. none of these
ans: A

28. Electrons (mass m, charge −e) are accelerated from rest through a potential difference V
and
are then deflected by a magnetic field

� B that is perpendicular to their velocity. The radius of

the resulting electron trajectory is:
A. (
�
2eV/m)/B
B. B√2eV /m
C. (
�
2mV/e)/B
D. B√2mV /e
E. none of these
ans: C