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Physics 102 Exam 2
4 April 2003 Prof L. Weinstein

Please give a short explanation for all of your non-numerical answers (unless otherwise noted). Show your work for all numerical answers.

Useful numbers: Atmospheric pressure = $10^5$ Pa = 100,000 N/m$^2$

Absolute zero = 0 K = -273$^o$ C = -459$^o$ F

Density of water = 1000 kg/m$^3$

Density of air at room temperature = 1.2 kg/m$^3$

Speed of Sound in Air at room termperature and pressure = 340 m/s

Speed of Light in vacuum = $3*10^8$ m/s

1. (No explanation needed) Light will always travel from one place to another along a path of least
   1. distance
   2. time
   3. effort
   4. expense
   5. complication
   Time.  - note that this problem is from a later chapter so I did not grade it.
   

2. The shortest plane mirror in which you can see your entire image is (hint: draw a picture for your explanation)
   1. half your height
   2. about 1/3 your height
   3. about 3/4 your height
   4. equal to your height
   5. dependent on the distance between you and the mirror
   
   half your height.  - note that this problem is from a later chapter so I did not grade it.
   
   

3. You look at a yellow light source. If this light is broken up into its component colors (eg: using diffraction glasses), you will see that it contains
   1. only yellow light
   2. lots of different colors ranging from red through green
   3. either one, it depends on the light source
   4. need more information
   
   Yellow light can be either spectral yellow (remember ROYGBIV) or a combination of red and green.  I gave 5 points for answer (2) and only counted the question as 5 points on the test (since that darn sodium lamp had a little bit of red and green along with the dominant yellow lines).
   
   
   
   
   

4. (no explanation needed) Which one of the colors below has the longest wavelength?
   1. red
   2. orange
   3. yellow
   4. green
   5. blue

5. What is the frequency of a blue photon of wavelength 440 nm (nano-meters)?

   f = c/lambda
   c = 3*10^8 m/s
   lambda = 440 nm = 440 * 10^(-9) m
   f = 3*10^8 m/s /  440 * 10^(-9) m = 6.82 * 10 ^(14) Hz
   
   
   

6. A mechanic is testing an air conditioner by running it on a workbench in an isolated room. What happens to the temperature of the room?
   1. it increases
   2. it decreases
   3. it stays the same
   4. need more information
   The air conditioner pumps heat from one side of the AC unit to the other (which is usually outside the room).  In this case, it pumps heat from the room back into the room.  This by itself would do nothing.  However, the AC uses electrical energy to do the pumping.  This energy is also converted to heat, raising the temperature of the room.
   
   
   
   
   
   

7. What input energy is required if an engine performs 200 J of work and exhausts 400 J of heat to the cold reservoir?

   If the output energy is 200 J of work plus 400 J of heat equals 600 J, then the input energy must also be 600 J.
   
   
   
   

8. We can treat people's legs as pendulums to analyze how they walk. Which legs take longer to swing back and forth, those of tall people or short people?
   1. tall
   2. short
   3. both the same
   4. need more information
   
   longer pendulums have longer periods because they have a larger rotational inertia.
   
   
   

9. What is the efficiency of an ideal heat engine with a boiler at 200$^o$ C that exhausts its heat at room temperature (27$^o$ C)?

   effi = [T(hot) - T(cold)] / T(hot) = (473 K - 300 K) /  473 K = 0.37
   
   You have to convert the temperature to Kelvin.
   
   

10. You stand just to the side of the low point of a child's swing and always push the child in the same direction. The period of the swing is 3 s. You push the swing at even intervals. Which intervals will make the swing go very high (resonance)? Circle all that apply.
    1. pushing once every 1 s
    2. pushing once every 2 s
    3. pushing once every 3 s
    4. pushing once every 4 s
    5. pushing once every 5 s
    6. pushing once every 6 s
    7. none of the above
    
    You need to push at the same point in each cycle.  The swing will return to the same point in the cycle once each period or once every three seconds.  If you push once every 3 seconds, you will push once a cycle.  If you push once every 6 seconds, then you will push once every two cycles.  Both will work.
    
    
    

11. (no explanation needed) When you yell at a friend, are the air molecules that strike his ear the same ones that left your lungs?
    1. Yes
    2. No
    The disturbance (the sound wave) reaches your friend's ear, not the air molecules. 
    

12. (no explanation needed) When you see your friend, are the photons that reach your eyes the same ones that bounced off her?
    1. Yes
    2. No

13. If you mix cyan and magenta paint, what color paint will result? (ie: if you shine white light on the mixture, what color will it appear?)

    magenta paint reflects red and blue light.  It absorbs green light
    cyan paint reflects blue and green light.  it absorbs red light
    therefore the mixture absorbs green and red light and reflects only BLUE light.
    
    
    
    

14. Draw diagrams to show the standing wave pattern for the first three harmonics of a rope fixed at both ends.

    
    
    
    
    
    
    
    
    
    
    

15. A pipe organ has pipes of many different lengths. Which ones make the highest notes?
    1. the shortest ones
    2. the middle ones
    3. the longest ones
    4. need more information
    shortest pipes have the shortest wavelength standing waves which have the highest frequency or pitch.
    
    
    
    

16. Your real estate agent assures you that extra traffic from the new shopping mall will only increase noise from cars by about 15%, from 60 dB to 70 dB. What does this imply about the number of cars using the street?
    1. It will increase by less than 15%
    2. It will increase by about 15%
    3. It will increase by more than 15%
    4. need more information
    
    A 10 dB increase corresponds to a factor of 10 increase in the intensity of the noise.  Therefore you will have about 10 times more cars to make 10 times as much noise.
    
    
    

17. A green cloth is illuminated by magenta light. What color does it appear?

    BLACK.  Green pigment absorbs red and blue light.  It  reflects only green light.  Magenta light contains red light and blue light.  Therefore all the magenta light is absorbed and none is reflected.