1© Manhattan Press (H.K.) Ltd. 11.7 Radar speed trap.

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1© Manhattan Press (H.K.) Ltd Radar speed trap

2 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 191) Radar speed trap Application of Doppler effect - radar speed trap (emits microwaves from hand-held speed gun in short bursts)

3 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 191) Radar speed trap - Moving vehicle (moving source towards speed gun) reflects incident wave - Speed gun receives the wave - Emitted wave and reflected wave are mixed - Velocity (v) are calculated by computer Go to Example 9 Example 9

4 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.1 Nature of sound waves 1. (a) All sounds are produced by the vibrations of objects. (b) Energy from the vibrating source is sent through the surrounding medium in the form of sound waves. (c) The frequency of the sound waves is the same as that of the vibrating source.

5 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.1 Nature of sound waves 2. (a) Compressions and rarefactions are formed along the path of the sound waves. The distance between two successive compressions (or rarefactions) is the wavelength of the sound wave. (b) The pressure is higher than normal at compression, while the pressure is lower than normal at rarefaction.

6 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.2 Properties of sound 3. The human ear responds to sounds in the frequency range 20 Hz to Hz. Frequencies above this range are called ultrasonics. Frequencies below this range are called infrasonics.

7 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.2 Properties of sound 4. Properties of sound: (a) Loudness: The loudness of a sound is determined by two factors, the intensity of the sound reaching the ear and the sensitivity of the listener’s ears. where a is the amplitude and r is the distance from the source.

8 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.2 Properties of sound 4. Properties of sound: (b) Pitch: The pitch of a sound is the sensation that the sound produces in a listener. The pitch of a sound is higher if the frequency of the sound is higher. (c) Quality: The quality of a note refers to its waveform.

9 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.2 Properties of sound 5. In practice, for a sound to appear twice as loud to the ear, its intensity must be increased by a factor of ten. For this reason, a logarithmic scale is used to measure intensity level. The decibel scale: where I is the sound intensity.

10 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.2 Properties of sound 6. (a) The minimum intensity of sound that the normal human ear can hear is about 1 ×10 −12 W m −2 at frequency of 1 kHz. It is called the threshold of hearing. (b) A sound of intensity 1 W m –2 or higher will cause pain to our ears. It is called threshold of pain.

11 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.3 Noise 7. Noises are sound which have irregular waveforms. They are unwanted, disturbing or even harmful. 8. The degree of damage on our hearing depends on the sound intensity level as well as the duration of exposure.

12 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.4 Speed of sound 9. The speed of sound waves in solid is: where E is the Young modulus and ρ is the density. 10. The speed of sound is the highest in solid and the lowest in gas.

13 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 193) 11.5 Doppler effect 11. The apparent change in the pitch of frequency of a source of sound when there is relative motion between the source and the observer is known as the Doppler effect. 12. If the source and observer approach each other, the apparent frequency is higher. Conversely, the apparent frequency is lower if they are moving away from each other.

14 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 194) 11.6 Doppler effect in light 13. The Doppler effect in light is used to determine whether a distant star or comet is approaching or moving away from the earth. The speed of the moving star can also be estimated.

15 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 194) 11.7 Radar speed trap 14. An application of Doppler effect is the radar speed trap. (a) Microwaves are emitted from a hand- held gun in short bursts. The emitted wave and the reflected wave are then superposed and beats are formed. (b) The beat signal is then passed to a computer built in the speed gun. From the beat frequency, the computer calculates the velocity of the vehicle and displays it on a screen.

16 © Manhattan Press (H.K.) Ltd Radar speed trap (SB p. 194)

17 © Manhattan Press (H.K.) Ltd. End

18 © Manhattan Press (H.K.) Ltd. Q: Q: A source of radiowaves of frequency f moves towards an object at speed. In a radar speed trap, 3 cm microwaves are used. If the waves are reflected from a vehicle travelling at 54 m s −1, find the frequency of the beats produced when the emitted and reflected waves are superposed together. Solution 11.7 Radar speed trap (SB p. 192)

19 © Manhattan Press (H.K.) Ltd. Solution: Return to Text 11.7 Radar speed trap (SB p. 192)