PHYS 243 Grade Distribution on Exam 4

PHYS 243 grade distribution on Exam 4

930 sect / 1030 sect / Total
0-9 / 1 / 2 / 3
10-19 / 5 / 3 / 8
20-29 / 8 / 13 / 21
30-39 / 20 / 14 / 34
40-49 / 13 / 9 / 22
50-59 / 10 / 14 / 24
60-69 / 4 / 8 / 12
70-79 / 7 / 7 / 14
80-89 / 2 / 2 / 4
90-99 / 0 / 1 / 1

The median grade on test 4 was around 43, though the average was somewhat higher. The answers with explanations appear below.

______

(a) (11pts) A block floats in water with 80% of its volume submerged. If the block is pushed down completely submerging it, what would be its upward acceleration when you let go of it?

Note that different versions of the test use different numbers on parts a and b of this problem. If block initially 80% submerged then: , but since the buoyant force and the weight are equal when the object is initially floating, we have . This means that the object’s density is therefore given by . By Newton’s 2nd law, when the object is submerged, we have: , which can be rewritten as: . Solving for a, we get = 1.23 m/s^2 (Obviously if your version gave different input numbers your answer would be different!)

(b) (11pts) A wire of length one meter is placed under tension. If the wire is made to vibrate in its 3rd harmonic at a frequency of 60 Hz, find the speed of the waves that travel along this wire.

(c) (10 pts) If a sound level meter measures 100 dB at a distance 10 meters from a point source, at what distance from the source would the meter register 80 dB?

The easy way: Use the rule that for every drop of 10 dB the intensity is reduced by a factor of 10. In going from 100 to 80 dB, the intensity must therefore drop by two factors of 10 or a factor of 100. This will occur if the distance increases by a factor of 10, because I varies as 1/r^2. Thus, r becomes 100 meters.

_____1. At what depth below the surface of a lake would a diver experience a total pressure of two atmospheres? (1 Atm = 1.01 x 10^5 N/m^2, and the density of water is 1000 kg/m^3) (a) 10.3 m (b) 20.6 m (c) 27.3 m (d) 38.4 m

Just use and solve for y

_____2. Two water tanks are filled to the same depth with water. Tank “a” has a bottom surface area of 2 m^2 and tank “b” has a bottom area of 4 m^2. Which statement about the forces and pressures at the bottom of the tanks is correct?

(a) Fa < Fb, and Pa < Pb(b).Fa < Fb, and Pa < Pb.

(c) Fa < Fb, and Pa = Pb.(d) Fa = Fb, and Pa = Pb.

P only depends on depth & fluid density, so they have the same P. F is proportional to area of bottom

______3. When a body of mass of 100 grams is suspended in water from a spring scale, the scale reads 25 grams. What is the density of the body, assuming that water has a density of 1 g/cm^3? (a) 0.25 g/cm^3 (b) 0.75 g/cm^3 (c) 1.3 g/cm^3 (d) 4.0 g/cm^3

But Therefore , or

______4. A simple pendulum has a period of 4.63 seconds at a place on the earth where the acceleration of gravity is 9.82 m/s^2. At a different location the period increases to 4.64 seconds. What is the value of g at this second point?

(a) 9.78 m/s^2 (b) 9.82 m/s^2 (c) 9.86 m/s^2 (d) Cannot be determined

Without actually doing any calculation the answer should be obvious, because a slightly smaller g would make the pendulum swing slower (longer period).

______5. At the lowest point in its swing, the speed of a pendulum bob is vo. If the length of the string had been slightly longer, and the pendulum were started from the same initial angle with the vertical, the speed of the pendulum bob as it passed through the lowest point in its swing would have been ___ vo.

(a) less than (b) more than (c) equal to (d) incomparable with

With a slightly longer string, the loss in PE at the lowest point will be a bit more, so the gain in KE will be a bit more.

______6. A wind of speed 10 m/s blows across a flat roof of area 20 m^2. Assuming an air density of 1.0 kg/m^3, what is the upward force on the roof according to the Bernouilli Principle?

(a) 730 N(b) 100 N(c) 267 N(d) 1000 N

As was shown in lecture, the Bernouilli equation in this case simplifies to give a pressure , and a force .

______7. According to the Doppler Effect, if you were to move away from a speaker at 5% the speed of sound you would hear a sound whose frequency is ______by _____

(a) raised, 5% (b) decreased, 5% (c) raised, 10% (d) decreased, 10%

Follows immediately from Doppler equation using vo=0.05 v

______8. A mass oscillates at the end of a spring of length L with a period of one second. If the length of spring is now cut in half and the amount of mass is doubled, the mass will be found to oscillate with a period of (a) 0.5 s, (b) 1 s, (c) 2 s, (d) 4 s

If the spring length is halved it will stretch only half as much given the same applied force, therefore its k is doubled. Since m is also doubled there is no change in the formula for the period.

______9. A pipe joins onto a second pipe having a third the diameter, and a fluid flows under pressure through both of them. The mass flow rate in the narrow pipe will be ______times that in the wide one, and the flow velocity will be ______times that in the wide one.

(a) 1, 9 (b) 1, 3 (c) 9, 1 (d) 3, 1 (e) none of them

Mass flow rates are the same, by the equation of continuity, which requires . Hence the velocity in the narrow pipe must be 9x greater since its cross sectional area is only a ninth that in the wide pipe.

______10. Which statement is false?

(a) Sound waves are longitudinal waves.

(b) Sound waves cannot travel in vacuum.

(d) Sound waves travel faster when the air is colder.

D is false based on the equation for the speed of sound in air versus temperature.

______11. The radius of an artery is reduced to 70% its original radius due to cholesterol. If the pressure gradient is doubled, the flow rate of blood through the artery is _____ of the original flow rate.

(a) 62%(b) 75%(c) 21%(d) 48%

Using the equation , if r changes to 0.7r and is doubled, then Q becomes 0.48 times the original value.

______12. The Tacoma Narrows bridge collapsed several months after it was built because on the day of the collapse the wind happened to have a speed that caused oscillations whose frequency ______the ______of the bridge

(a) matched, natural frequency (b) exceeded, natural frequency

Essentially the definition of resonance, as discussed extensively in class

______13. Infrasonic sound waves have a much ______wavelength and a much ______frequency than ultrasonic waves.

(a) smaller, lower(b) smaller, higher(c) larger, lower(d) larger, higher

Infrasonic waves are below the minimum audible frequency 20 Hz, while ultrasonic waves are above the highest frequesncy we can hear.

______14. In an organ pipe open at ______, the ______harmonics are missing.

(a) one end, odd(b) both ends, odd(c) one end, even(d) both ends, even

Just draw a diagram for the first few permitted standing waves, and write down the formula for the wavelengths and frequencies, as was done in lecture.

______15. If you double the amplitude of vibration of a mass oscillating on a spring its mechanical energy is ______times the original, and its period is ____ times the original.

(a) 4, 1(b) 4 ,2(c) 2, 1(d) 2, 2

Based on and

______16. A boat is floating in the middle of a small lake with a large boulder in it. When the boulder is thrown overboard and sinks to the bottom, the level of the lake ______. (Hint: This demo was done in class. Just think about the amount of water the boulder displaces.)

(a) rises slightly(b) falls slightly(c) remains unchanged(d) cannot be determined

This was demonstrated and explained in lecture.

______17. At a certain instant during its oscillation a mass on a spring has 75% kinetic energy and 25% potential energy. The displacement of the mass at this instant is ______times the amplitude. (a) 0.50 (b) 0.36 (c) 0.71 (d) 0.85

Just use: and solve for x.

PHYS 243, Exam 4

Fall 2002 semester

Name ______mult. Choice______

(please print)Problem ______

Total ______

The multiple choice questions count four points each, for a total of 68 points (out of a total of 100). Skip the very difficult questions, and come back to them if you have time. Work must be shown to receive credit on the problem. You are expected to observe the GMU Honor Code. Please do not leave the room until the end of the period. When I announce the exam is over please stop writing immediately and turn your exam over until it is collected.

(a) (11pts) A block floats in water with 60% of its volume submerged. If the block is pushed down completely submerging it, what would be its upward acceleration when you let go of it?

(b) (11pts) A wire of length one meter is placed under tension. If the wire is made to vibrate in its 3rd harmonic at a frequency of 100 Hz, find the speed of the waves that travel along this wire.

(c) (10 pts) If a sound level meter measures 100 dB at a distance 10 meters from a point source, at what distance from the source would the meter register 80 dB?

Put your answers on the line next to each question number.

______1.. Infrasonic sound waves have a much ______wavelength and a much ______frequency than ultrasonic waves.

(a) smaller, lower(b) smaller, higher (c) larger, lower(d) larger, higher

______2. In an organ pipe open at ______, the ______harmonics are missing.

(a) one end, odd(b) both ends, odd(c) one end, even(d) both ends, even

______3. If you double the amplitude of vibration of a mass oscillating on a spring its mechanical energy is ______times the original, and its period is ____ times the original.

(a) 4, 1(b) 4 ,2(c) 2, 1(d) 2, 2

______4. At what depth below the surface of a lake would a diver experience a total pressure of two atmospheres? (1 Atm = 1.01 x 10^5 N/m^2, and the density of water is 1000 kg/m^3) (a) 10.3 m (b) 20.6 m (c) 27.3 m (d) 38.4 m

______5. Two water tanks are filled to the same depth with water. Tank “a” has a bottom surface area of 2 m^2 and tank “b” has a bottom area of 4 m^2. Which statement about the forces and pressures at the bottom of the tanks is correct?

(a) Fa < Fb, and Pa < Pb(b).Fa < Fb, and Pa < Pb.

______6. When a body of mass of 100 grams is suspended in water from a spring scale, the scale reads 25 grams. What is the density of the body, assuming that water has a density of 1 g/cm^3? (a) 0.25 g/cm^3 (b) 0.75 g/cm^3 (c) 1.3 g/cm^3 (d) 4.0 g/cm^3

______7. A simple pendulum has a period of 4.63 seconds at a place on the earth where the acceleration of gravity is 9.82 m/s^2. At a different location the period increases to 4.64 seconds. What is the value of g at this second point?

(a) 9.78 m/s^2 (b) 9.82 m/s^2 (c) 9.86 m/s^2 (d) Cannot be determined

______8. At the lowest point in its swing, the speed of a pendulum bob is vo. If the length of the string had been slightly longer, and the pendulum were started from the same initial angle with the vertical, the speed of the pendulum bob as it passed through the lowest point in its swing would have been ___ vo.

(a) less than (b) more than (c) equal to (d) incomparable with

______9. A wind of speed 10 m/s blows across a flat roof of area 20 m^2. Assuming an air density of 1.0 kg/m^3, what is the upward force on the roof according to the Bernouilli Principle?

(a) 730 N(b) 100 N(c) 267 N(d) 1000 N

______10. According to the Doppler Effect, if you were to move away from a speaker at 5% the speed of sound you would hear a sound whose frequency is ______by _____

(a) raised, 5% (b) decreased, 5% (c) raised, 10% (d) decreased, 10%

______11. A mass oscillates at the end of a spring of length L with a period of one second. If the length of spring is now cut in half and the amount of mass is doubled, the mass will be found to oscillate with a period of (a) 0.5 s, (b) 1 s, (c) 2 s, (d) 4 s

______12. A pipe joins onto a second pipe having a third the diameter, and a fluid flows under pressure through both of them. The mass flow rate in the narrow pipe will be ______times that in the wide one, and the flow velocity will be ______times that in the wide one.

(a) 1, 9 (b) 1, 3 (c) 9, 1 (d) 3, 1 (e) none of them

______13. Which statement is false?

(a) Sound waves are longitudinal waves.

(b) Sound waves cannot travel in vacuum.

(d) Sound waves travel faster when the air is colder.

______14. The radius of an artery is reduced to 70% its original radius due to cholesterol. If the pressure gradient is doubled, the flow rate of blood through the artery is _____ of the original flow rate.

(a) 62%(b) 75%(c) 21%(d) 48%

______15. The Tacoma Narrows bridge collapsed several months after it was built because on the day of the collapse the wind happened to have a speed that caused oscillations whose frequency ______the ______of the bridge

(a) matched, natural frequency (b) exceeded, natural frequency

(a) rises slightly(b) falls slightly(c) remains unchanged(d) cannot be determined

PHYS 243, Exam 4

Fall 2002 semester

Name ______mult. Choice______

(please print)Problem ______

Total ______

(a) (11pts) A block floats in water with 30% of its volume submerged. If the block is pushed down completely submerging it, what would be its upward acceleration when you let go of it?

(b) (11pts) A wire of length one meter is placed under tension. If the wire is made to vibrate in its 3rd harmonic at a frequency of 120 Hz, find the speed of the waves that travel along this wire.

(c) (10 pts) If a sound level meter measures 100 dB at a distance 10 meters from a point source, at what distance from the source would the meter register 80 dB?

Put your answers on the line next to each question number.

______1. The Tacoma Narrows bridge collapsed several months after it was built because on the day of the collapse the wind happened to have a speed that caused oscillations whose frequency ______the ______of the bridge

(a) matched, natural frequency (b) exceeded, natural frequency

______2. A boat is floating in the middle of a small lake with a large boulder in it. When the boulder is thrown overboard and sinks to the bottom, the level of the lake ______. (Hint: This demo was done in class. Just think about the amount of water the boulder displaces.)

(a) rises slightly(b) falls slightly(c) remains unchanged(d) cannot be determined

______3. At a certain instant during its oscillation a mass on a spring has 75% kinetic energy and 25% potential energy. The displacement of the mass at this instant is ______times the amplitude. (a) 0.50 (b) 0.36 (c) 0.71 (d) 0.85

______4. Infrasonic sound waves have a much ______wavelength and a much ______frequency than ultrasonic waves.

(a) smaller, lower(b) smaller, higher (c) larger, lower(d) larger, higher

______5. In an organ pipe open at ______, the ______harmonics are missing.

(a) one end, odd(b) both ends, odd(c) one end, even(d) both ends, even

______6. If you double the amplitude of vibration of a mass oscillating on a spring its mechanical energy is ______times the original, and its period is ____ times the original.

(a) 4, 1(b) 4 ,2(c) 2, 1(d) 2, 2

______7. At what depth below the surface of a lake would a diver experience a total pressure of two atmospheres? (1 Atm = 1.01 x 10^5 N/m^2, and the density of water is 1000 kg/m^3) (a) 10.3 m (b) 20.6 m (c) 27.3 m (d) 38.4 m

______8. Two water tanks are filled to the same depth with water. Tank “a” has a bottom surface area of 2 m^2 and tank “b” has a bottom area of 4 m^2. Which statement about the forces and pressures at the bottom of the tanks is correct?

(a) Fa < Fb, and Pa < Pb(b).Fa < Fb, and Pa < Pb.

______9. When a body of mass of 100 grams is suspended in water from a spring scale, the scale reads 25 grams. What is the density of the body, assuming that water has a density of 1 g/cm^3? (a) 0.25 g/cm^3 (b) 0.75 g/cm^3 (c) 1.3 g/cm^3 (d) 4.0 g/cm^3

______10. A simple pendulum has a period of 4.63 seconds at a place on the earth where the acceleration of gravity is 9.82 m/s^2. At a different location the period increases to 4.64 seconds. What is the value of g at this second point?

(a) 9.78 m/s^2 (b) 9.82 m/s^2 (c) 9.86 m/s^2 (d) Cannot be determined

______11. At the lowest point in its swing, the speed of a pendulum bob is vo. If the length of the string had been slightly longer, and the pendulum were started from the same initial angle with the vertical, the speed of the pendulum bob as it passed through the lowest point in its swing would have been ___ vo.

(a) less than (b) more than (c) equal to (d) incomparable with

______12. A wind of speed 10 m/s blows across a flat roof of area 20 m^2. Assuming an air density of 1.0 kg/m^3, what is the upward force on the roof according to the Bernouilli Principle?

(a) 730 N(b) 100 N(c) 267 N(d) 1000 N

______13. According to the Doppler Effect, if you were to move away from a speaker at 5% the speed of sound you would hear a sound whose frequency is ______by _____

(a) raised, 5% (b) decreased, 5% (c) raised, 10% (d) decreased, 10%

______14. A mass oscillates at the end of a spring of length L with a period of one second. If the length of spring is now cut in half and the amount of mass is doubled, the mass will be found to oscillate with a period of (a) 0.5 s, (b) 1 s, (c) 2 s, (d) 4 s

______15. A pipe joins onto a second pipe having a third the diameter, and a fluid flows under pressure through both of them. The mass flow rate in the narrow pipe will be ______times that in the wide one, and the flow velocity will be ______times that in the wide one.

(a) 1, 9 (b) 1, 3 (c) 9, 1 (d) 3, 1 (e) none of them

______16. Which statement is false?

(a) Sound waves are longitudinal waves.

(b) Sound waves cannot travel in vacuum.

(d) Sound waves travel faster when the air is colder.

______17. The radius of an artery is reduced to 70% its original radius due to cholesterol. If the pressure gradient is doubled, the flow rate of blood through the artery is _____ of the original flow rate.

(a) 62%(b) 75%(c) 21%(d) 48%

PHYS 243, Exam 4

Fall 2002 semester

Name ______mult. Choice______

(please print)Problem ______

Total ______

The multiple choice questions count four points each, for a total of 68 points (out of a total of 100). Skip the very difficult questions, and come back to them if you have time. Work must be shown to receive credit on the problem. You are expected to observe the GMU Honor Code. Please do not leave the room until the end of the period. When I announce the exam is over please stop writing immediately and turn your exam over until it is collected.

(a) (11pts) A block floats in water with 40% of its volume submerged. If the block is pushed down completely submerging it, what would be its upward acceleration when you let go of it?

(b) (11pts) A wire of length one meter is placed under tension. If the wire is made to vibrate in its 3rd harmonic at a frequency of 80 Hz, find the speed of the waves that travel along this wire.

(c) (10 pts) If a sound level meter measures 100 dB at a distance 10 meters from a point source, at what distance from the source would the meter register 80 dB?

Put your answers on the line next to each question number.

(a) matched, natural frequency (b) exceeded, natural frequency