Wednesday, 10 September 2014

Vibrations and waves 2

Speed of sound in air = 340 m/s                Speed of light in a vacuum = 300 000 000 m/s

1. What is meant by:
(a) a transverse wave;
(b) a longitudinal wave?
Give one example of each type.

2 What is meant by:
(a) the amplitude of a wave;
(b) the wavelength of a wave;
(c) the frequency of a wave?

3. A water wave has a wavelength of 3 in and a frequency of 4 Hz. What is its velocity?

4. A ‘cello string vibrates at 64 Hz. If a lump of plasticine was stuck onto the middle of the string, what would you expect to happen to the frequency of vibration?

5. The ratio of the length of two pipes on an organ is 2:1. What is the ratio of the note produced by the shorter pipe to that produced by the longer pipe?

6. A timekeeper sees the puff of smoke from the starting pistol but starts his watch when he hears the sound of the gun. If he was 100 m from the start and his stopwatch showed 10.4 s when the runner passed the finish, what was the correct time for the race?

7. A guitar string is ‘stopped’ in the middle and when plucked it gives a note with a frequency of 200 Hz. What is the frequency of the ‘unstopped’ string?

8. What is meant by a harmonic?

9. Someone says that electronic music played by a computer does not sound like ‘real’ music. What would you say to this?

10. A wave moves up a steadily sloping beach. What would you expect to happen to the velocity of the wave?

11. Stroboscopes are used to
(a) check the timing of cars;
(b) check the speed of record player decks.

Explain how they do this.
12. A flashing light stroboscope is used to look at / a spinning black disc which has one white arrow painted on it. If the disc is spinning at 100 revs/second what will you see when the stroboscope flashes at:
(a) 100 times/second
(b) 200 times/second
(c) 50 times/second
(d) 25 times/second
(e) 300 times/second
Explain each answer carefully.

13. Using a flashing rate of 100 times/second what will you see if the disc in question 13 is gradually allowed to slow down?

14. A hand held stroboscope is a wooden disc / with 12 slots cut in it. It is used to look at a golf ball rolling along the bench at a steady speed of 72 cm/s. If the disc rotates 2 twice every second:
(a) how many glimpses of the ball are seen each second;
(b) what is the time interval between each glimpse;
(c) how far will the golf ball have rolled between glimpses?

15. A wagon wheel seen on a film appears to be spinning backwards very slowly. Explain this.

16. The wagon’s speed now alters so that the wheels look stationary. If the film has 20 frames per second and the wheel has 10 spokes, how many times per second is the wheel rotating?

17. A film is taken of the growth of a plant. One picture is taken every 2 days. If you want to show the film so that the plant appears to grow 2900 times as fast as normal, how many pictures must be shown per hour?

18. In a hot concert hall a flute may sound sharp. Why is this?