Explain why (a) To keep a piece of paper horizontal, you should blow over, not under, it (b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers (c) The size of the needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection (d) A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel (e) A spinning cricket ball in air does not follow a parabolic trajectory
(a)
When we blow over the piece of paper, then the velocity of air increases. As a result From the Bernoulli’ theorem, the pressure over it decreases whereas the atmospheric pressure below remains the same.
Thus, to keep the paper horizontal, we should blow over the paper not under it.
(b)
From the equation of continuity,
Area × velocity = constant
The above equation shows that, for an opening with smaller area velocity is more as compared to the opening with larger area.
Thus, when we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers.
(c)
For a constant height, the Bernoulli’s theorem is expressed as,
P + 1 2 ρ v 2 = constant
In this equation, the pressure P occurs with a single power whereas the velocity occurs with a square power. Therefore, the velocity has more effect compared to the pressure. For this reason needle of the syringe controls flow rate better than the thumb pressure exerted by the doctor.
(d)
This is because of principle of conservation of momentum. While the flowing fluid carries forward momentum, the vessel gets a backward momentum.
(e)
A spinning cricket ball would have followed a parabolic trajectory if there been no air. But because of air, the Magnus effect takes place. Due to the Magnus effect the spinning cricket ball deviates from its parabolic trajectory. Thus, a spinning cricket ball does not follow a parabolic trajectory.
(a)
When we blow over the piece of paper, then the velocity of air increases. As a result From the Bernoulli’ theorem, the pressure over it decreases whereas the atmospheric pressure below remains the same.
Thus, to keep the paper horizontal, we should blow over the paper not under it.
(b)
From the equation of continuity,
The above equation shows that, for an opening with smaller area velocity is more as compared to the opening with larger area.
Thus, when we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers.
(c)
For a constant height, the Bernoulli’s theorem is expressed as,
In this equation, the pressure
(d)
This is because of principle of conservation of momentum. While the flowing fluid carries forward momentum, the vessel gets a backward momentum.
(e)
A spinning cricket ball would have followed a parabolic trajectory if there been no air. But because of air, the Magnus effect takes place. Due to the Magnus effect the spinning cricket ball deviates from its parabolic trajectory. Thus, a spinning cricket ball does not follow a parabolic trajectory.
