### Spring 2011 Physics 101 Hour Exam 3(25 questions)

The grading button and a description of the scoring criteria are at the bottom of this page.

True-false questions are worth 2 points each, three-choice multiple choice questions are worth 3 points each, five-choice multiple choice questions are worth 6 points each. The maximum possible score is 114; the average score was 80.1; the median score was 81. The exam period was 90 minutes. Click here to see page1 page2 of the formula sheet that came with the exam.

Unless told otherwise, you should assume that the acceleration of gravity near the surface of the earth is 9.8 m/s2 downward and ignore any effects due to air resistance.

Choose the closest number to the correct answer when a numerical answer is required>.

Assume that a pendulum is near the earth's surface.

### QUESTION 1*

This question and the following one relate to the same situation:

A sound wave of frequency f travels from water (speed of sound is 1500 m/s) to steel (speed of sound is 5600 m/s). What is the ratio of the wavelength of this sound in water λwater to that in steel λsteel ?

(a)   λsteel / λwater = 0.25
(b)   λsteel / λwater = 0.73
(c)   λsteel / λwater = 1.00
(d)   λsteel / λwater = 2.53
(e)   λsteel / λwater = 3.73

### QUESTION 2*

λsteel is 35 cm for this sound. What is the frequency of the sound?

(a)   f = 7 kHz
(b)   f = 12 kHz
(c)   f = 16 kHz
(d)   f = 19 kHz
(e)   f = 21 kHz

### QUESTION 3***

A uniform string is stretched between a transducer and a smooth peg. The string is stretched by a hanging block of mass M. The distance between the transducer and the peg is a fix distance L.

The number of nodes in the figure is 4 (including both ends). We now change the block of mass M to another block of mass m, while keeping the frequency of the transducer the same. This produces a standing wave with 3 nodes on the string (including both ends). What is the required mass m of the new block?

(a)   m = 9 M / 4
(b)   m = 7 M / 4
(c)   m = 3 M / 2
(d)   m = 5 M / 4
(e)   m = 3 M / 4

### QUESTION 4*

This question and the following one relate to the same situation:

A siren gives a loudness β7 when it is 7 m away and β9 when it is 9 m away.

Find the difference β7 - β9.

(a)   β7 - β9 = 0.98 dB
(b)   β7 - β9 = 1.58 dB
(c)   β7 - β9 = 2.18 dB

### QUESTION 5**

A single siren 9 meters away makes a sound of loudness β9. What is the loudness β of 3 sirens, identical to the first one, placed at the same location?

(a)   β = β9 + 4.8
(b)   β = 3 β9
(c)   β = 4.8 β9

### QUESTION 6**

You drive a car at a constant speed along a road. On the roadside is a stationary siren whose frequency observed by a bystander is f0. When you are approaching the siren, you hear the frequency fb = 740 Hz and when you are leaving the siren, you hear the frequency fa = 690 Hz. Assume that the speed of sound is 330 m/s. What is the frequency f0 of the siren?

(a)   671 Hz
(b)   690 Hz
(c)   702 Hz
(d)   715 Hz
(e)   740 Hz

### QUESTION 7**

On a 1 m square plate is a hole of area A = 0.3 m2 at T = 200 K. The plate is made of a material whose linear thermal expansion coefficient is α = 13 × 10-6 K-1. What is the increase ΔA of the area of the window at T = 300 K compared with that at T = 200 K ?

(a)   ΔA = 5.3 × 10-4 m2
(b)   ΔA = 7.8 × 10-4 m2
(c)   ΔA = 9.3 × 10-4 m2
(d)   ΔA = 11.2 × 10-4 m2
(e)   ΔA = 13.5 × 10-4 m2

### QUESTION 8**

This question and the following one relate to the same situation:

In a container of volume V = 0.5 m3 is 0.8 kg of an ideal gas. Its pressure is P = 1. 25 × 105 Pa at temperature T = 290 K.

What is the molecular mass M of the molecules making the gas?

(a)   M = 22.3 amu
(b)   M = 30.8 amu
(c)   M = 33.5 amu
(d)   M = 41.1 amu
(e)   M = 49.0 amu

### QUESTION 9**

What is the ratio of the root mean square velocity v290 of the molecules at T = 290 K and the root mean square velocity v580 of the molecules at T = 580 K ?

(a)   v580 / v290 = 1.21
(b)   v580 / v290 = 1.31
(c)   v580 / v290 = 1.41
(d)   v580 / v290 = 1.71
(e)   v580 / v290 = 2.00

### QUESTION 10*

This question and the following three relate to the same situation:

An object of mass m is hanging from a vertical spring of spring constant k (= 20 N/m) near the surface of the earth. In equilibrium, the spring is stretched by 10 cm relative to the relaxed length of the spring. The spring is then compressed by 1 cm relative to the equilibrium position and is released into oscillation at time t = 0. The height of the object relative to the equilibrium height oscillates as shown below as a function of time.

Among the three time points marked A and B and C, when is the speed of the object the greatest?

(a)   A
(b)   B
(c)   C

### QUESTION 11*

Between the two time points marked A and B, when is the acceleration of the object the largest in magnitude?

(a)   A
(b)   B

### QUESTION 12*

What is the mass m of the object?

(a)   0.10 kg
(b)   0.15 kg
(c)   0.20 kg

### QUESTION 13**

Which of the following curves would best describe the height vs. time curve, if the mass of the object is quadrupled with all other quantities remaining the same?

(a)
(b)
(c)
(d)
(e)

### QUESTION 14**

This question and the following one relate to the same situation:

A pendulum hanging from the ceiling of an elevator is swinging with the period of 2 seconds when the elevator is at rest. Assume that the elevator is near the surface of the earth. Suddenly, the elevator undergoes vertical acceleration and the period of the pendulum has changed to 2.2 seconds.

What is the direction of acceleration?

(a)   upward
(b)   downward

### QUESTION 15**

What is the magnitude of acceleration?

(a)   1.4 m/s2
(b)   1.7 m/s2
(c)   2.0 m/s2
(d)   2.3 m/s2
(e)   2.6 m/s2

### QUESTION 16*

This question and the following one relate to the same situation:

A container is filled with water to the brim and a uniform block of unknown density is added to the container. The block floats with 75% of its volume immersed in water and the amount of water overflowed from the container is 1 liter (1 liter is 10-3 m3). Assume that the mass density of water is 1,000 kg/m3.

What is the mass density of the unknown material?

(a)   1,250 kg/m3
(b)   1,100 kg/m3
(c)   900 kg/m3
(d)   750 kg/m3
(e)   600 kg/m3

### QUESTION 17**

What is the weight of the block?

(a)   9.8 N
(b)   11.2 N
(c)   12.5 N
(d)   15.7 N
(e)   18.2 N

### QUESTION 18**

This question and the following one relate to the same situation:

A pendulum is made of a small weight of mass 1.5 kg attached to a string of length 2 m. The mass is released gently with the initial angle displacement of 5°.

How long does it take for the mass to reach its lowest point for the first time?

(a)   2.84 s
(b)   1.42 s
(c)   0.71 s

### QUESTION 19**

What is the maximum kinetic energy of the pendulum?

(a)   0.33 J
(b)   0.22 J
(c)   0.11 J

### QUESTION 20**

This question and the following one relate to the same situation:

An incompressible and non-viscous βfluid flows from left to right through a circular pipe that changes its radius from r1 to r2 between regions 1 and 2 (see figure, not to scale). The fluidic velocity in region 1 is v1 (= 3 m/s) and the fluidic velocity in region 2 is v2 (= 9 m/s). The density of fluid is 1,300 kg/m3.

What is the ratio between r2 and r1?

(a)   r2 / r1 =0.58
(b)   r2 / r1 =0.44
(c)   r2 / r1 =0.33

### QUESTION 21*

What is the magnitude of the pressure difference between the two regions?

(a)   |P2-P1| = 24,500 Pa
(b)   |P2-P1| = 35,300 Pa
(c)   |P2-P1| = 46,800 Pa
(d)   |P2-P1| = 57,900 Pa
(e)   |P2-P1| = 0 Pa

### QUESTION 22*

Yuri constructs a barometer as shown below using water as the fluid in order to measure the atmospheric pressure. He takes it to the top of a mountain. The column of water reaches the height h of 9.18 m. The water density is 1,000 kg/m3.

What is the atmospheric pressure measured by the barometer?

(a)   115,000 Pa
(b)   105,000 Pa
(c)   95,000 Pa
(d)   90,000 Pa
(e)   80,000 Pa

### QUESTION 23**

A horizontal pipe becomes narrow and then widens as the fluid inside flows to the right as shown below.

Which one of the following statements is true?

(a)   The pressure of the fluid is everywhere the same because the average height of the fluid is the same everywhere the same.

(b)   The pressure of the fluid is smallest on the right end of the pipe because its diameter is the largest.

(c)   The pressure of the fluid is the smallest in the narrowed section because the velocity is the largest there.

(d)   The pressure of the fluid is the largest in the narrowed section because the velocity is the largest there.

(e)   None of the above statements is true.

### QUESTION 24*

When you blew on a pipe to make some sound, one end was open. Despite this, there was some reflection between the open end of the instrument and the surrounding air.

(T)   True
(F)   False

### QUESTION 25*

You blow into a pipe and record the output. The pressure variation as a function of time looks like:

You then blow into another pipe and the output sound is lower in pitch. Which of these is a possible pressure variation recorded just outside the pipe as a function of time?

(a)   A
(b)   B
(c)   C

Scoring of True-False (T,F) Questions:
• If you mark the correct answer, you get 2 points.
• If you mark the wrong answer or if you mark neither answer, you get 0 points.
Scoring of Multiple Choice I (a,b,c) Questions:
• If you mark the correct answer, you get 3 points.
• If you mark a wrong answer or if you mark none of the answers, you get 0 points.
Scoring of Multiple Choice II (a,b,c,d,e) Questions:
• If you mark one answer and it is the correct answer, you get 6 points.
• If you mark two answers and one of them is the correct answer, you get 3 points.
• If you mark three answers and one of them is the correct answer, you get 2 points.
• If you do not mark the correct answer, or if you mark more than three answers, or if you mark none of the answers, you get 0 points.