250+ TOP MCQs on Fluids Mechanical Properties – Streamline Flow | Class 11 Physics

Physics Multiple Choice Questions on “Fluids Mechanical Properties – Streamline Flow”.

1. Velocity has to be the same at the same horizontal level in case of steady laminar flow.
a) True
b) False

Answer: b
Clarification: Velocity can be different at the same horizontal level if the area of the cross section at the two points is different. This is according to the equation of continuity, A₁v₁ = A₂v₂, where A is area and v is velocity.

2. A tube of uniform cross section always has water flowing through it. It is kept vertical in such a way that water enters from top and leaves from the bottom. If the speed at a point A below the opening is ‘v’, what will be the speed at a point B vertically below A such that the distance between A & B is ‘2h’?
a) v
b) (sqrt{v^2 + 4gh})
c) (sqrt{v^2 + 2gh})
d) v/2

Answer: a
Clarification: According to the equation of continuity, the velocity at B will be the same as that at A since the area of the cross section is the same at both points. You may be confused by thinking that gravity will increase its velocity but the pressure will also decrease downwards and therefore net effect will be zero change in velocity.

3. Choose the correct option regarding a streamline.
a) Speed, not velocity, at all points of a streamline is same
b) Two streamlines can intersect in case of laminar flow
c) There is no friction between streamlines in case of steady flow
d) In a given streamline, velocity of a point can vary with time

Answer: d
Clarification: If we consider a streamline in a pipe of varying diameter we can say that speed of different points in that streamline are different. The tangent to a streamline gives the direction of velocity at that point, so two streamlines cannot intersect as at the point of intersection we will not be able to draw a single tangent. In case of steady flow across a pipe, there could be streamlines next to each other having different velocities and hence there will be friction between the streamlines which we call viscosity. If flow is unsteady, but not turbulent, the velocity of a point in a streamline can change. For eg: If we slowly increase the speed of a tap flow will be laminar and speed of a point (on a streamline) will increase with time.

4. On which of these following options is the continuity equation based?
a) Work energy theorem
b) Law of conservation of energy
c) Conservation of mass
d) Conservation of momentum

Answer: c
Clarification: The continuity equation is based on the conservation of mass. We consider that fluid is incompressible (constant density) and say that mass of fluid passing through 2 different regions at the same time is the same.
∴ density X Vol₁ = density X Vol₂.
∴ Vol₁ = Vol₂.
∴ A₁v₁Δt = A₂v₂Δt
∴ A₁v₁ = A₂v₂.

5. In which one of the following cases can the equation of continuity be used?
a) Compressible flow
b) Incompressible flow
c) Turbulent flow
d) Viscous flow

Answer: b
Clarification: For using the continuity equation we assume that the flow is incompressible, laminar and non-viscous. The option that says compressible flow implies that fluid’s density can be changed. So it’s incorrect. The option that says turbulent flow is incorrect as we have to assume flow to be laminar for the equation of continuity to hold. The option that says viscous flow is also incorrect because we assume flow to be frictionless for the equation of continuity to hold.

6. Streamlines of steady flows can be curved.
a) True
b) False

Answer: a
Clarification: A streamline is a curve to which tangent at every point gives the velocity of that point. The flow of water through a bent pipe can be steady, and when we observe a streamline in it, it will be curved.