50 Conceptual Questions with Answers

Bernoulli’s Theorem, Terminal Velocity, Viscous Force and Upthrust

1. What does Bernoulli’s theorem state?
   Answer: Bernoulli’s theorem states that for an ideal fluid in steady flow, the sum of pressure energy, kinetic energy and potential energy per unit volume remains constant along a streamline.

2. Write Bernoulli’s equation.
   Answer: (P + \frac{1}{2}\rho v^2 + \rho gh = constant)

3. What happens to pressure when fluid speed increases?
   Answer: Pressure decreases when fluid speed increases.

4. Why does air pressure decrease above an aeroplane wing?
   Answer: Air moves faster over the upper surface of the wing, so pressure becomes lower there.

5. How does Bernoulli’s theorem explain lift on an aeroplane?
   Answer: Faster air above the wing creates lower pressure, while slower air below creates higher pressure. This pressure difference produces lift.

6. Why does a paper strip rise when air is blown over it?
   Answer: Blowing air increases speed above the paper, reducing pressure there. Higher pressure below pushes the paper upward.

7. What type of fluid is assumed in Bernoulli’s theorem?
   Answer: The fluid is assumed to be ideal, incompressible, non-viscous and in steady flow.

8. Why is Bernoulli’s theorem not perfectly valid for real fluids?
   Answer: Real fluids have viscosity, so some mechanical energy is lost as heat.

9. What is the role of density in Bernoulli’s equation?
   Answer: Density affects both kinetic energy term and potential energy term of the fluid.

10. Why does water flow faster from a narrow pipe section?
    Answer: According to the equation of continuity, when area decreases, velocity increases.

11. What happens to pressure in the narrow part of a pipe?
    Answer: Pressure decreases in the narrow part because velocity increases.

12. What is a venturimeter used for?
    Answer: A venturimeter is used to measure the rate of flow of a liquid through a pipe.

13. Why does a spray pump work?
    Answer: Fast-moving air creates low pressure, which draws liquid upward and sprays it.

14. Why do two boats moving close together tend to come closer?
    Answer: Water flows faster between them, pressure decreases there, and outside pressure pushes them together.

15. What is terminal velocity?
    Answer: Terminal velocity is the maximum constant velocity attained by a body falling through a fluid.

16. Why does a falling object attain terminal velocity?
    Answer: Because viscous force and buoyant force increase until the net force becomes zero.

17. What is the acceleration of a body at terminal velocity?
    Answer: Acceleration becomes zero.

18. At terminal velocity, what is the net force on the body?
    Answer: Net force is zero.

19. Does terminal velocity depend on the radius of the falling sphere?
    Answer: Yes, terminal velocity is proportional to the square of the radius of the sphere.

20. Why does a raindrop fall with nearly constant speed after some time?
    Answer: It reaches terminal velocity due to air resistance.

21. Why does a small raindrop fall slower than a large raindrop?
    Answer: Smaller radius gives smaller terminal velocity.

22. What is the formula for terminal velocity of a small sphere in a viscous fluid?
    Answer: (v_t = \frac{2r^2(\rho – \sigma)g}{9\eta})

23. What does (\eta) represent in terminal velocity formula?
    Answer: (\eta) represents the coefficient of viscosity of the fluid.

24. What happens to terminal velocity if viscosity increases?
    Answer: Terminal velocity decreases.

25. What happens to terminal velocity if the density of the body increases?
    Answer: Terminal velocity increases.

26. What happens if the density of the body equals the density of the fluid?
    Answer: Terminal velocity becomes zero because effective weight becomes zero.

27. Why does a steel ball fall faster in water than in glycerine?
    Answer: Glycerine has higher viscosity, so it offers greater resistance.

28. What is viscous force?
    Answer: Viscous force is the internal frictional force between different layers of a fluid moving with different velocities.

29. Write Newton’s law of viscosity.
    Answer: (F = \eta A \frac{dv}{dx})

30. On what factors does viscous force depend?
    Answer: It depends on viscosity, area of contact and velocity gradient.

31. What is velocity gradient?
    Answer: Velocity gradient is the rate of change of velocity with distance between fluid layers.

32. What is the SI unit of coefficient of viscosity?
    Answer: Pascal-second or (N s m^{-2}).

33. Why is honey more viscous than water?
    Answer: Honey has stronger internal friction between its layers.

34. Why does viscosity decrease in liquids when temperature increases?
    Answer: Higher temperature weakens intermolecular forces, reducing viscosity.

35. Why does viscosity increase in gases when temperature increases?
    Answer: Molecular motion increases, causing more momentum transfer between gas layers.

36. What is Stokes’ law?
    Answer: Stokes’ law states that the viscous force on a small sphere moving through a fluid is (F = 6\pi \eta r v).

37. When is Stokes’ law valid?
    Answer: It is valid for small spherical bodies moving slowly through a viscous fluid.

38. Why is viscous force opposite to motion?
    Answer: Because it resists relative motion between the body and the fluid.

39. What is upthrust?
    Answer: Upthrust is the upward force exerted by a fluid on a body immersed in it.

40. What is buoyant force?
    Answer: Buoyant force is another name for upthrust.

41. State Archimedes’ principle.
    Answer: When a body is immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced by it.

42. Why does a body feel lighter in water?
    Answer: Because water exerts an upward buoyant force on the body.

43. What determines whether a body floats or sinks?
    Answer: It depends on the relation between the weight of the body and the upthrust acting on it.

44. When does a body float?
    Answer: A body floats when its weight is equal to the upthrust.

45. When does a body sink?
    Answer: A body sinks when its weight is greater than the upthrust.

46. Why does a wooden block float on water?
    Answer: Its density is less than water, so it can displace enough water to balance its weight.

47. Why does an iron nail sink in water?
    Answer: Its density is greater than water, so its weight is greater than the upthrust.

48. Why does a ship made of iron float?
    Answer: A ship has a hollow structure, so its average density becomes less than water.

49. What is the formula for upthrust?
    Answer: Upthrust = weight of displaced fluid = (V\rho g)

50. How are upthrust and terminal velocity connected?
    Answer: During falling in a fluid, upthrust acts upward along with viscous force. Terminal velocity is reached when weight is balanced by upthrust and viscous force.