HOW THEY BUILD YOUR FLAT?

Today, multistorey buildings are common. To build a sky scrapper, we have to lift concrete, steel and other materials to a certain height. So, first they build a tower crane. One can see this crane near any tall building under constuction. First, let us understand the principle of this crane.

Take a meter scale and hang it at the 50th cm. Take a 150 gm weight and hang it 10 cm (at 40 cm) from the centre of the scale (50cm). Now the moment of this weight is 'weight * distance' from centre.
Moment of 150 gm = 150*10 = 1500 gm cm. If you hang only this weight, the scale will tilt to one side and collapse.
Take another weight of 50 gm and hang it 30 cm from the centre on the other side (at 80cm).
Moment of 50 gm weight = 50 * 30 = 1500 gm centimeter.
The two moments of these two weights are equal. Hence the weights will balance and the scale will be stable.
This 'principle of moments' used in tower crane, levers, gears, rotational motions etc.

The tower crane is fixed on a concrete base. The mast is tightened to the base with heavy anchor bolts. There is a steel arm on the mast. It has opertor's cabin and counter wights on one side.
Let us suppose a crane has capacity 300 tonne meters. It means, the 10 tonne weight should be hanged at 30 meters distance or 15 tonne weight should be hanged at 20 meter distance. Then the counter weight will balance the 'weight to be lifted' properly.
Normally heavy wights arre hanged at smaller distance. The height of the mast is increased as the building groves. Again usually, the maximum length of arm (jib) is 18 meters and the maximum weight that can be lifted is 18 tonnes.
Safty measures are built in. If wrong weight or distance is chosen, a warning alarm will go. Normally weight and distance is marked on the jib (arm).

OTHER EXAMPLE OF MOMENTS:
1. If you hold the door at the edge, opening and closing will be easy.
2. In a scissor, cutting arm is long and holding arm is short.
3. See saw
4. 'Moment' of inertia or mass is important in rotatory motions.
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