ANSWER:

1. The trucks that carry heavy loads have double tyres in the rear wheels in order to maintain balance.

The engine of the trucks are connected to the rear tyres. In case of heavy loads, the rear tyres should balance the complete load as the front tyre will be balancing the load of the front part of the truck which are engine and other attributes. So the weight of the load will be carried by the rear tyres only. If single rear tyres are used there may be chances of getting burst being unable to carry the heavy load, so double tyres are used so that the load can be distributed equally and the truck can be maintained.

2. Reply: The nose bleeding at higher altitudes is due to reduced oxygen content at higher altitudes.

The concentration of oxygen in atmosphere decreases with increase in the altitude. So the absence of oxygen in the atmosphere will lead to disturbance in the system of human body as the oxygen pressure in the body will not be equal to the pressure of oxygen in the atmosphere. So the oxygen present in tissues will increase the blood pressure of the human body mostly in nose as the tissues in nose are mostly used for intake of oxygen. So the decrease in oxygen content in the atmosphere lead to drier air which when the human being intake it leads to bursting of tissues in nose releasing blood from nose.

3. Reply: Lifting a box under water is easier than lifting it in air because of the existence of buoyant force in water.

The molecules underwater exhibit an upward thrusting force which has the tendency to push the objects in water to upward direction. This is the reason when we swim underwater we get tired easily and loose large amount of calories as we have work against the upward buoyant force so work is done by us. In this case, if we have to lift a box it will be easier as the direction of force acting on the box while lifting will be parallel to the direction of buoyant force. But in air, there is no buoyant force so the lifting of box should be done opposite to the direction of gravitational pull acting on the box. Thus the lifting of box in underwater will be easier than lifting it in the air.