1-Why buses and trucks have double wheels at the back? 2-Why poters place a round piece of cloth on their heads? 3-Why dams are made stronger and thicker at the bottom than the top? 4-Why the pressure inside an aeroplane is maintained at normal ground level atmospheric pressure?

1.

Buses and trucks have double wheels at the back because these are heavy vehicles with most of the loads resting towards the back. Also, the engine that drives the vehicle is attached to the back wheels so the double back wheels provides better hold on the ground. Double wheels are  not used in the front since that would make the steering uncomfortable.

2.

The cloth on the head of porters acts as cushion and increase the contact area between the load and the head. The increased area reduces the pressure on the head due to the heavy loads. This is according to the relation, P = F/A. An increase in area will decrease the pressure due to a force (load).

3.

In any building the lower portion is made stronger and thicker to provide strength to the building. Similar is the case with dams. The thick bottom simply makes the dam strong.

4.

Air pressure is low at high altitude. So, pressure inside an aeroplane is maintained similar to pressure at the ground. Since, low pressure can cause our blood vessels to rupture causing bleeding from nose, eyes etc.