Q. Oil spreads over the surface of water, whereas, water does not spread over the surface of oil. Why?
Ans. The surface tension of the water is more than of oil. Therefore, when oil is poured over water, the greater value of surface tension of water pulls the oil in all direction and as such it spreads on the water. On the other hand, when water is poured over oil, it does not spread over it because the surface tension of oil being less than that of water, it is not able to pull water over it.
Q. It is advised not to stand near a running train. Why?
Ans. When fast moving train passes on a rail, than the velocity of air streams in between the rail and the person standing near rail will be very large as compared to the velocity of air streams on the other side of person away from the rail. According to Bernoulli’s theorem, the pressure of air will become low in between person and rail and is high on the other side of person. As a result of the pressure difference, a thrust acts on the person which may push the person towards rail side and the person may meet with an accident
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.
- 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.
(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 cantre? 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.