Metals when present near a magnetic field create disturbance in the field. This is similar to the waves created by a stone thrown into a stream of water. Metal detectors are equipped with sensitive electronic circuitry to detect disturbances created by metal objects in the magnetic field. The hand held metal detectors have two coils which create magnetic fields of similar strengths. Whenever a metal object comes near any of the coils it disturbs the magnetic field there. This upsets the balance between the two fields and alerts an electronic circuitry. Some detectors do not have any coils, but instead sense the disturbances created by metal objects in the natural field present in the surroundings. The door-frame type of metal detectors works this way. They have a series of coils in the frame which continuously monitor the surrounding magnetic field. Any disturbance caused in the field is sensed and an alarm set off.
Locks are nothing but special latching devices in which the secured bolt is released only by a specially designed tool, the key. In an ordinary door latch, a bolt is pushed into a hole in the door-frame, and is then secured by turning the key. A similar principle works in a padlock or the portable lock. However, in a padlock, a projection called tumble-bolt is pushed into a notch in the shockle and made secure by a special arrangement of levers. The levers are placed at different heights and the tumble moves only when all the levers are moved at once. Notches of different heights are made on the key such that they are in alignment with the levers of different heights. Thus, when the key is turned, all the levers are operated together and the tumble-bolt is moved into the groove in the shockle or out of it. By different combinations of the levers of various heights, each lock is made to open with a specific key.
The operation of the parachute is based on the simple principle of balancing two opposing forces. There are two forces that act on any falling object —gravity and air resistance.
Unlike an aeroplane, helicopters have moving wings in the shape of spinning rotor blades. Air currents passing over the upper and lower surfaces of the spinning blades create low pressure above it and generate lift. So, a helicopter does not have to taxi on a runway to get the lift. The lift can be controlled by varying the pitch (angle) of the rotor blades. By increasing the pitch of the main rotor blades lift is increased and the craft climbs. Lowering the pitch of the blades reduces lift and gravity causes the helicopter to descend. Similarly, if the pilot holds the rotors pitch in such a way that the blades produce just enough lift to counteract gravity, the helicopter remains stationary in mid-air.
Aeroplanes fly by a combination of power from the engines and lift provided by the wings. These are shaped in such a way that air flowing along the upper surface takes a longer path than the air flowing along the lower surface. As a result when an aircraft moves forward, the air flowing above the wings moves faster than the air flowing below the wing, creating a low pressure above the wing according to Bernoulli’s law. This difference in pressure below and above the wings causes lift. As the speed of the aircraft on the runway increases, the lift also increases, eventually overcoming the downward force of gravity. The aircraft also uses the thrust created by its engine to climb and after reaching a certain height cruises along in a horizontal direction.