Conceptual Questions of chapter 6 (Fluid Dynamics)
Q.1) From the top of a tall building, you drop two table-tennis balls, one filled with air and the other
with water. Which ball reaches terminal velocity first and why?
Answer:
The terminal velocity of an object is given by;
This shows that terminal velocity depends on density, size and shape of the object as well as the medium
through which it travels. The two tennis balls are of same size and shape but their densities are different.
From above relation we can see that
VT ∝ 𝜌
Hence the terminal velocity of air filled tennis ball will be reached first because its density is lighter than the
tennis ball filled water. In other words the upward drag force becomes equal to mg sooner for lighter tennis
ball.
Q.2) Why can a squirrel jump from a tree branch to the ground and run away undamaged, while a
human could break a bone in such a fall?
Answer:
Both squirrel and human fall under the action of gravity i.e. the force acting on them is given by;
F = mg
Since g is constant and mass of human is greater than that of mass of squirrel so a human would experience
a large force as compared to that of squirrel. Hence he could break a bone unlike squirrel.
Q.3) How does the terminal speed of a parachutist before opening a parachute compare to the
terminal speed afterward? Why is there a difference?
Answer:
Before opening of parachute the weight is bigger force than air resistance so the skydiver accelerates
downward unless it reaches the point where air resistance increases and become equal to weight i.e. the
terminal velocity is achieved.
When parachute is opened the terminal velocity is reduced because now air resistance increases
dramatically: it is much greater than the weight force, so the skydiver slows down. The air resistance force
reduces until it becomes equal to weight and second terminal velocity is achieved.
Thus the difference in air resistance results in difference in terminal velocities before and after opening of
parachute.
Q.4) You can squirt water over a greater distance by placing your thumb over the end of a garden
hose, than by leaving it completely uncovered. Explain how this works?
Answer:
From equation of continuity the crossectional area and the velocity of fluid are inversely related to each
other to keep their product constant. Hence when we place our thumb over the end of a garden hose, its
crossectional area is decreased and so, the velocity of water is increased due to which it can squirt over
greater distance as compared to when it is left completely uncovered.
Q.5) Why does smoke rise faster in a chimney on a windy day?
Answer:
As the wind blows across the top of the chimney, so the pressure over there will be less than that of inside.
And Bernoulli’s principle for fluids tells us that it is nature of fluid that it always flows from area of high
pressure to the area of lower pressure.
When a breeze is blowing, it will create a pressure difference between inside and outside of the chimney,
which in turn causes faster rise of smoke in chimney.
Q.6) Two boats moving in parallel paths close to one another risk colliding. Why?
Answer:
When the two boats moving in parallel path come close to one another the velocity of streamlines becomes
very large between them as compared to velocity on the other sides. Owing to this the pressure between
