Name:_____________________ Recit Teacher:__________________

__________________________ Date: ________________________

-1 of 2-

Physics 71 TWHFW 2nd

semester 2013-14: Recitation Quiz 16 INSTRUCTIONS: Answer the following questions clearly and legibly. Show your complete solutions and box you final answer. Any act of

dishonesty such as cheating shall be subjected to disciplinary actions.

1-2. A thin 75.0-cm wire has a mass of 16.5g. One end is tied to a nail and the other end is attached to a screw that can

be adjusted to vary the tension in the wire.

1. To what tension (in newtons) must you adjust the screw so that a transverse wave of wavelength 3.33cm

makes 875 vibrations per second?

smmxHzfv /1.29)1033.3)(875( 2

Since mkgm

kg

L

m/0220.0

750.0

0165.0 Therefore the tension is:

NsmmkgvF 6.18)/1.29)(/0220.0(2

2. How fast would this wave travel?

smmxHzfv /1.29)1033.3)(875( 2

3-4. A piano with mass 3.00g and length 80.0cm is stretched with a tension of 25.0N. A wave with frequency 120.0Hz

and amplitude 1.6mm travels along the wire.

3. Calculate the average power carried by the wave.

Note that: L

m and f 2 ; everything is given in the problem to get the average power:

WAFPac 223.02

1 22

4. What happens to the average power if the wave amplitude is halved?

Pave is proportional to A2 so halving the amplitude quarters the average power to 0.056W.

5-6. Two trains whistles, A and B, each have a frequency of 392Hz. A is stationary and B is moving toward

the right (away from A) at a speed of 35.0m/s. A listener is between the two whisles and is moving toward

the right with the speed 15.0m/s. No wind is blowing.

5. What is the frequency from A heard by the listener?

The positive direction is from listener to source fs = 392Hz. vS = 0; vL = -15.0m/s.

HzHzsm

smsmf

vv

vvf S

S

LL 375)392(

/344

/0.15/344

The distance between whistle A and the listener is increasing, and for whisle A; fL<fS.

6. What is the frequency from B heard by the listener?

vS = +35.0m/s; vL = +15.0m/s.

HzHzsmsm

smsmf

vv

vvf S

S

LL 371)392(

/0.35/344

/0.15/344

The distance between whistle B and the listener is also increasing, and for whistle B; fL<fS.

Name:_____________________ Recit Teacher:__________________

__________________________ Date: ________________________

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7-9. One string of a certain musical instrument is 75.0cm long and has a mass of 8.75g. It is being played in

a room where the speed of sound is 344m/s.

7. What is the frequency and speed of the waves on the string so that when vibrating in its second

overtone, it produces sound wavelenght 3.35cm?

The standing waves have wavelengths n

Ln

2 ;

for fundamental n = 1; for second overtone n=3; therefore mL

500.03

2

fv

The frequency is Hzxv

f 41003.1

So that the speed of the wave is: smxfv /1015.5 3

8. To what tension must you adjust the string?

From /Fv ;

with mkgxm

kgx

L

m/1017.1

750.0

1075.8 23

we can calculate the tension as: NxvF 52 1010.3

9. What frequency sound does this string produce in its fundamental mode of vibration?

For standing waves the frequencies: 1nffn . So for second overtone (n=3): Hzxf

f 311 1043.3

3

10-11. Light is a wave, but not mechanical wave. The quantities that oscillate are electric and magnetic fields. Light

visible to humans has wavelengths between 400nm (violet) and 700nm (red), and all travels through vacuum at speed

c = 3.00x108m/s.

10. What are the (upper and lower) limits of the frequency of visible light?

fv

With nm400

Hzxmx

smxcf 14

9

8

1050.710400

/1000.3

With nm700

Hzxmx

smxcf 14

9

8

1029.410700

/1000.3

11. What are the (upper and lower) limits of the period of visible light? Could you time a single light vibration

using a stopwatch?

fT

1

With nm400

sxf

T 151033.11

With nm700

sxf

T 151033.21 T is very small, hence cannot be measured using the stopwatch.