Question Corner

Measuring speed of vehicles

QUESTION: How does the speed (of vehicles) measuring device used by the traffic police work?

G. Srinivasan, Thanjavur, T.N.

ANSWER 1: Chronometers are used to determine the speed of the vehicle with a hand held stopwatch. No radar, no loop just a plain and simple digital stop watch, and two points of a known distance apart. Start and stop switches are to be manually actuated when the vehicle crosses the points.

The speed will be displayed instantly. The second method is by using a Piezo Sensors, this method uses set of 3 rubber strips with a specific distance apart are build across the road or buried in the road. The time between compressions is measured to give the resulting speed of the vehicle. If the speed exceeds the camera associated with the system will photograph the vehicle.

In another method two beams of invisible infrared beams are emitted across the given traffic lane. The elector optic head directs the beams to two retro-reflectors mounted on the road surface. Every vehicle passing between the electro optic head and the reflectors breaks the beams and triggers the unit computer to measure the speed. If a vehicle exceeds speed limit, the high- resolution camera photographs the front or rear of the vehicle including its license plate.

In another method, Infra red diodes are pulsed at certain frequencies and to create radar beams. This beam is emitted via parabolic reflector at 15 degree spread and directed towards the traffic. The emitters are shielded to prevent external interference.

They normally use K band in a frequency 24GHZ. The associated computer will measure speed and triggers camera to capture the image of erring vehicles.

K. Kamalakkannan, Doha, Qatar.

ANSWER 2: This works on the phenomenon of Doppler effect. When a sound source and an observer are in relative motion with respect to the medium in which the waves propagate, the frequency of waves observed is different from the frequency of sound emitted by the source.

This phenomenon is called Doppler effect. This is due to the wave nature of sound propagation.

The principle of Doppler effect is used to detect the speed of automobiles by traffic police. An electromagnetic wave of constant frequency is emitted by a source attached to the police van. The wave is reflected by the moving vehicle which in turn acts as an moving source.

S. Kishore Kumar, Madurai, T.N.

Sound of thunder

QUESTION: How is sound produced in thunder?

K.A.Narayanan,Kanyakumari, T.N.

ANSWER: Warm, wet air surges upwards into the sky and cools dramatically forming thunderstorms. Some of the water inside the clouds freezes and strong air currents make the ice and water droplets bump together. This knocks tiny charged particles called electrons from the ice and so there is a build-up of electrical charge.

This charge is released by a stroke of lightning. The lightning heats the air around it to an incredible 30,000 oC (54,000 oF), five times hotter than the surface of the Sun. This heat causes the air to expand very fast; i.e., faster than the speed of sound. It is this which causes the crash of thunder.

If the thunderstorm is overhead, we can hear thunder and see lightning simultaneously. If it is not overhead, we can see lightning first, as light travels much faster than sound. By counting the seconds between the lightning and thunder and dividing by three we get the distance to the storm in kilometres and dividing by five we get the distance in Miles. Dark thunderclouds are formed on hot and humid days. A thundercloud is usually about 5 Kms (3 Miles) across and 8 Kms (5 Miles) high.

T. V. Jayaprakash, Palakkad, Kerala

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