## When air is blown in between two balls suspended close to each other, they are attracted towards each other . Why ?

Q.When air is blown in between two balls suspended close to each other, they are attracted towards each other. Give reason.

Ans.     When air is blown in between two balls suspended close to each other with high speed, a low pressure is created between the balls which is much less than the atmospheric pressure beyond the balls. Due to this, balls are attracted towards each other. This is according to Bernoulli’s theorem.

## Blood pressure in human beings is greater at the feet than at the brain.Why ?

1. Explain why

(a) The blood pressure in human beings is greater at the feet than at the brain.

(b) Atmospheric pressure at a height of about 6km decreases to nearly half of its value at the sea level, though the height of the atmosphere is more than 100 km.

(c) Hydrostatic pressure is a scalar quantity even though pressure is force divided by area.

Ans.

(a)   The height of the blood column is quite large at feet that at the brain.

Consequently, the blood pressure in human beings is greater at the feet than at the brain.

(b)   Density of air decreases very rapidly with increase in height and reduces to nearly half its value at the sea level at a height of about 60 km. After 6 km, the density of air deceases but rather very slowly. This is the reason that at a height of about 6 km, a atmospheric pressure reduces to nearly half of its value at sea level.

(c)   Force is a vector quantity but pressure is a scalar quantity. According to Pascal’s law, pressure is transmitted equally in all directions i.e.., direction is not associated with the pressure. Hence, it is a scalar quantity.

## Can a satellite be in an orbit in a plane not passing through the earth’s centre?

Can a satellite be in an orbit in a plane not passing through the earth’s centre? Explain .

Ans.     The centripetal force required for the orbital motion of the satellite is provided by the gravitational force of attraction. Gravitational force is a central force, i.e.., it passes through the centre of mass of the earth and the satellite. Hence, the plane of orbit of the satellite has to pass through the earth’s centre.

## Why a spacecraft consume more fuel in going from earth to moon than it does on the return trip ?

A spacecraft consume more fuel in going from earth to moon than it does on the return trip. Comment on this.

Ans.     In going from earth to moon, the spacecraft has to do more work against the greater gravitational attraction of the earth. For the return journey, moon’s gravitational force is much less, hence less work is done and less fuel is consumed.

## Why are space crafts usually launched from west to east?

Why are space crafts usually launched from west to east? Why is it more advantageous to launch rockets in the equatorial plane?

Ans.     Earth rotates on its axis from west to east. A satellite launched from west to east will have the advantage of the additional velocity of the earth’s rotation. The effect is maximum  at the equator, hence it is most advantageous to launch the satellite from west to east on the equatorial plane.