Class 12 Physics Fluid statics Notes

UNIT 1
CLASS 12 PHYSICS • MECHANICS

Fluid Statics

Chapter 3

Fluid Statics (Hydrostatic)

Fluid

The substance which can flow from one point to another point is called fluid. Example ⇒ liquid & gas.

Hydrostatics (Fluid Statics)

The study of fluid at rest is called hydrostatics.

Density

Mass per unit volume is called density. Its unit is kg m−3.

Relative Density (Specific Gravity)

Relative density of a substance is the ratio of its density to the density of water at 4°C.

Relative density (Sr) = ρ / ρw
where, ρ = density of substance
ρw = density of water = 1 g/cm3 = 1000 kg/m3

Pressure

Force per unit area is called pressure. Its unit is N/m2 or pascal (Pa).
In fluid, pressure (P) = hρg

Derivation of Pressure in Liquid

Let us consider a liquid of density ‘ρ’ contained in a vessel of cross-sectional area ‘A’ up to height ‘h’ as shown in figure.

Liquid of density rho in a vessel of base area A and height h ρ h A
Liquid in a vessel of cross-sectional area A and height h

Now, total mass of liquid:

m = volume (V) × density (ρ)
= Area (A) × height (h) × density (ρ)
= Ahρ

Weight of liquid is given by:

W = mg
= Ahρg

So, the normal force on the bottom of the vessel is:

F = W
= Ahρg

Finally, the pressure exerted by liquid at the bottom of vessel is:

P = F/A
= Ahρg / A
P = hρg

This is the required relation for pressure due to liquid and this relation shows that pressure due to liquid is directly proportional to:

  1. density of liquid,
  2. height of liquid in vessel,
  3. acceleration due to gravity at that place.

Principle of Flotation

Principle of flotation states, “The weight of a floating body is equal to the weight of liquid displaced.”

OR

“A body is said to be floating in liquid if upthrust acting on the body is greater or equal to the weight of the body.”

When a body is immersed in a liquid there are three cases:

i) Weight of body is greater than upthrust (W > U)

Body sinking because weight is greater than upthrust U W
W > U
W > U
Vρg > Vρ1g
ρ > ρ1

In this case, body sinks and set up at bottom.

ii) Weight of body is equal to upthrust

Body in liquid with weight equal to upthrust U W
W = U
W = U
Vρg = Vρ1g
ρ = ρ1

In this case, the body lies inside liquid with upper surface on the surface of liquid.

iii) Weight of body is less than upthrust

Body floating partly above the liquid surface U W
W < U
W < U
Vρg < Vρ1g
ρ < ρ1

In this case, a part of body lies inside the liquid and remaining part outside the liquid surface.

Upthrust, Archimedes’ Principle, Pascal’s Law & Centre of Buoyancy

Upthrust

The upward force exerted by a fluid on an object which is completely or partially immersed in fluid is called upthrust or buoyancy.

Archimedes’ Principle

It states, “When a body fully or partially immersed in a fluid, it experiences an upthrust which is equal to the weight of fluid displaced by the body.”
Upthrust = Weight of fluid displaced

Pascal’s Law

It states that, “When a pressure is applied to an enclosed liquid then the pressure is equally transmitted to every portion of it.”

Centre of Buoyancy

The centre of gravity of the displaced liquid is called centre of buoyancy.
Floating body showing centre of gravity and centre of buoyancy on the same vertical line C.G. C.B. U mg
Centre of gravity and centre of buoyancy on the same vertical line

A floating body is said to be in equilibrium if centre of gravity and centre of buoyancy of the body lie in same vertical line.

Surface Tension

Cohesive and Adhesive Force

Cohesive force: The force of attraction between similar molecules is called cohesive force. e.g. force between water molecules.
Adhesive force: The force of attraction between dissimilar molecules is called adhesive force. e.g. force between water & glass molecules.

Surface Tension

The property by virtue of which liquid surface behaves as stretched membrane and tends to occupy the minimum surface area is called surface tension.

Mathematically: Surface tension is the force per unit length of an imaginary line drawn in the plane of liquid surface acting at right angle of this line.

If ‘F’ be the force acting on the imaginary line of length ‘l’, then:

T = F/l

Note: Surface-tension expressions for different objects

CaseSource conditionExpression
ObjectObject of length l on surface of liquidT = F/l
ObjectObject of length l inside the liquidT = F/(2l)
RingRing of perimeter 2πr on surfaceT = F/(2πr)
RingRing of perimeter 2πr inside liquidT = F/(4πr)
RectangleRectangle of perimeter 2(l+b) on surfaceT = F/[2(l+b)]
RectangleRectangle of perimeter 2(l+b) inside liquidT = F/[4(l+b)]
Clean schematic of object, ring and rectangle surface-tension cases l Object on surface 2πr Ring inside liquid l, b Rectangle in liquid
Representative surface-tension cases from the scanned notes

Surface Energy

The amount of workdone against surface tension per unit change in surface area is called surface energy.
Surface energy (σ) = Workdone in increasing surface area / Change in surface area
Rectangular wire frame ABCD with sliding side BC displaced by x A B C D T F x l
Fig: Surface energy

Let us consider a rectangular frame ABCD of wire in which BC can be slid horizontally. Suppose it is dipped in a soap solution such that a thin layer is deposited over an area. Due to surface tension (T), the layer tends to minimize surface area. Suppose force ‘F’ is applied perpendicularly to BC towards outwards. So, the membrane is extended by distance ‘x’ horizontally.

F = T × 2l
[Since the soap film has two surfaces in contact with air, so total length of wire = 2l]

So, workdone by this force F:

W = F × d
= F × x
W = T × 2l × x

Hence,

σ = Workdone / Change in area
= (T × 2l × x) / (2 × l × x)
σ = T

This surface energy is equal to surface tension.

Shape of Liquid Meniscus

Concave water meniscus and convex mercury meniscus in glass Water in glass concave meniscus F₍C₎ < F₍A₎ Mercury in glass convex meniscus F₍A₎ < F₍C₎
Water forms a concave meniscus; mercury forms a convex meniscus

i) Convex Meniscus

In convex meniscus, the cohesive force between mercury molecules is greater than the adhesive force between mercury molecules and glass molecules.

FA < FC

ii) Concave Meniscus

In concave meniscus, the adhesive force between mercury molecule and water molecule is greater than the cohesive force between water molecules.

FA > FC

Angle of Contact

The angle between tangent to the liquid meniscus at point of contact with the surface of solid inside the liquid surface is called angle of contact. For example, angle of contact for water glass is acute and angle of contact for mercury glass is obtuse.
Angle of contact in water and mercury θ Angle of contact in water θ Angle of contact in mercury
Angle of contact in water is acute; in mercury it is obtuse

Capillarity

The phenomena of rise or fall of the liquid in a capillary tube when dipped partially in it is called capillarity.

For example:

  • When a capillary tube with narrow bore is dipped in water, the level of water rises in capillary tube.
  • When same capillary tube is dipped in mercury, the level of liquid falls in capillary tube.
Capillary rise in water and capillary depression in mercury upward Water: capillary rise downward Mercury: capillary fall
Capillary rise in water and depression in mercury

Measurement of Surface Tension by Capillary Rise Tube

Capillary rise showing T, T cos theta, T sin theta, radius r and height h T T T cosθ T cosθ T sinθ T sinθ h r θ θ liquid, ρ
Fig: Capillary rise

Let us consider a capillary tube of radius ‘r’ is dipped in a liquid of density ‘ρ’. Let ‘h’ be the height above which liquid rise in the capillary tube. The surface tension acting tangentially at concave meniscus is equal and opposite to the relation. If ‘θ’ be angle of contact, then T can be resolved into two components—T sinθ along horizontal and T cosθ along vertical. The horizontal components cancel each other and vertical components add up together which pull liquid upwards.

The total vertical force acting upward throughout the circumference is given by:

F = T cosθ × 2πr   — (i)

This force pulls the liquid upwards until weight of rise liquid equals to upward force.

Now, volume of liquid in the tube above free surface of liquid is given by:

V = [Volume of cylinder of radius r & height h]
    + [Volume of cylinder of radius r & height r]
    − [Volume of hemisphere of radius r]
V = πr2h + πr3 − (2/3)πr3
V = πr2h + (1/3)πr3
V = πr2(h + r/3)

So, weight of liquid rise:

W = mass of liquid × g
= Vρg
= πr2(h + r/3)ρg   — (ii)

At equilibrium:

Total upward force = Weight of liquid rise
T cosθ × 2πr = πr2(h + r/3)ρg
T cosθ = (h + r/3)ρrg / 2
T = [(h + r/3)ρrg] / (2 cosθ)   — (iii)

Thus, knowing r, h, θ, ρ and g value, T can be calculated.

For narrow tube: h ≫ r/3, so h + r/3 ≈ h.

T = ρrgh / (2 cosθ)
h = 2T cosθ / (ρrg)

Numericals — Surface Tension & Capillarity

Q.1 — Capillary rise

A capillary tube of 0.3 m diameter is placed vertically inside a liquid of density 800 kgm−3, surface tension 5×10−4 Nm−1 and angle of contact 30°. Calculate the height to which the liquid rises in the capillary tube.

diameter (d) = 0.3 m
radius (r) = 0.3/2 = 0.15 m
density of liquid (ρ) = 800 kg/m3
surface tension (T) = 5×10−4 N/m
angle of contact (θ) = 30°
h = 2T cosθ / (ρrg)
h = [2(5×10−4) cos30°] / [800×0.15×10]
h = 7.2×10−7 m

Q.2 — Rectangular plate and surface tension

A rectangular plate of dimensions 6 cm by 4 cm and thickness 2 mm is placed with its largest face flat on the surface of water. Calculate the downward force on the plate due to surface tension assuming zero angle of contact. What is the downward force if the plate is placed vertical so that its longest side just touches the water?

Surface tension of water, T = 7.0×10−2 N m−1
l = 6 cm = 6×10−2 m
b = 4 cm = 4×10−2 m
t = 2 mm = 2×10−3 m

Downward force on the plate due to surface tension:

F1 = T × 2(l+b)
= 7×10−2 × 2(6+4)×10−2
F1 = 1.4×10−2 N

Again, downward force when the plate is placed vertical and its largest side just touches:

F2 = T × 2(l+t)
= 7×10−2 × 2(6×10−2 + 2×10−3)
F2 = 8.68×10−3 N

Q.3 — Breaking one drop into eight drops

Find the workdone required to break up a drop of water of radius 5×10−3 m into eight drops of water assuming isothermal condition.

Surface tension of water, T = 7.2×10−3 N/m
Radius of big drop (R) = 5×10−3 m
Number of drops (n) = 8
Weight/volume of big drop = weight/volume of small 8 drops
(4/3)πR3ρg = 8(4/3)πr3ρg
R3 = 8r3
R = 2r
r = R/2 = 2.5×10−3 m

Workdone = T × change in area

W = T[8×4πr2 − 4πR2]
= 7.2×10−3[4π(8r2 − R2)]
W = 2.26×10−6 J

Q.4 — Breaking one drop into one million droplets

Calculate the work done in breaking a drop of water of 2 mm diameter into million droplets of same size. T of water = 72×10−3 N/m.

D = 2 mm, R = 10−3 m
T = 72×10−3 N/m
Volume of big drop = volume of million drops
R3 = 106r3
R = 102r
r = R/100 = 10−5 m

Again, workdone is:

W = T × change in area of drop
= 72×10−3[106×4πr2 − 4πR2]
= 72×10−3×4(3.14)[106r2 − R2]
W = 8.95×10−5 J

Viscosity

The property by virtue of which a fluid layer opposes the relative motion of its different layer is called viscosity.

Generally: Viscosity means the frictional force acting between the layers of fluid that oppose the relative motion between them.

Newton’s Formula for Viscosity

Different liquid layers flowing over a fixed solid surface with velocity gradient v + dv v x dx F A fixed solid horizontal surface
Fig: Different layers of liquid flowing over the solid surface

Let us consider a liquid flowing over the fixed solid horizontal surface. The liquid flows in different layers parallel to the fixed surface. The layer in contact with the fixed surface is at rest while the velocity of other layer increases uniformly upward.

Let ‘A’ be the area of layers in contact and v & v+dv are the velocity of layer at distance x & x+dx from fixed surface respectively, then dv/dx is velocity gradient.

According to Newton’s law of viscosity, viscous force ‘F’ acting between two layers of liquid is:

i) directly proportional to area of contact:
F ∝ A   — (i)
ii) directly proportional to velocity gradient:
F ∝ dv/dx   — (ii)

Combining eqn (i) and (ii), then:

F ∝ A(dv/dx)
F = −ηA(dv/dx)

where ‘η’ is proportionality constant called coefficient of viscosity. The negative sign indicates that viscous force decreases the relative motion/velocity between two layers.

η = −F / [A(dv/dx)]
If A = 1 m2 and dv/dx = 1 s−1, then η = −F.

Thus, coefficient of viscosity is defined as the viscous force acting per unit area having unit velocity gradient.

Dimensional formula of coefficient of viscosity

η = F / [A(dv/dx)]
= [MLT−2] / ([L2][T−1])
[η] = [ML−1T−1]

Poiseuille’s Formula

Streamline flow of liquid through a capillary tube of radius r and length l P₁ P₂ l r
Fig: Streamline flow of liquid in capillary tube

Poiseuille concluded that the volume ‘V’ of the liquid flowing per second through a capillary tube is:

  1. directly proportional to the pressure difference between two ends: V ∝ P,
  2. directly proportional to the fourth power of radius of tube: V ∝ r4,
  3. inversely proportional to the coefficient of viscosity of liquid: V ∝ 1/η,
  4. inversely proportional to the length of capillary tube: V ∝ 1/l.

Combining equations (i), (ii), (iii) & (iv), we get:

V ∝ Pr4/(ηl)
V = kPr4/(ηl), where k = π/8
V = πPr4/(8ηl)

This is called Poiseuille’s formula.

Derivation of Poiseuille’s Formula by Dimensional Method

Consider a liquid through a capillary tube of radius r with length ‘l’ such that pressure difference between two end of tube is P = P1 − P2.

According to Poiseuille’s, the volume per second ‘V’ depends upon:

  1. the pressure gradient (P/l),
  2. radius of capillary tube (r),
  3. coefficient of viscosity (η).
In dimensional form:
V ∝ (P/l)a rb ηc
V = k(P/l)a rb ηc   — (i)
where k = π/8 called proportionality constant.

The dimensional equation of eqn (i) is:

[M0L3T−1] = [ML−2T−2]a[L]b[ML−1T−1]c

Equating dimensions:

a + c = 0
−2a + b − c = 3
−2a − c = −1
a = 1, c = −1, b = 4

Putting a = 1, b = 4, c = −1 in eqn (i):

V = (π/8)(P/l)r4η−1
V = πPr4/(8lη)

Which is Poiseuille’s formula.

Stoke’s Law

When a spherical body falls through liquid, the upthrust and viscous force act in upward direction and weight of the body acts in downward direction. With increase in velocity of the body, the viscous force also increases and at certain time when total upward forces equal to downward force, the body falls with constant velocity called terminal velocity.

Stoke’s law found that the viscous force (F) acting on a spherical body of radius ‘r’ moving with terminal velocity ‘v’ in a fluid of coefficient of viscosity ‘η’ is:
F = 6πηrv
Spherical body falling downward in liquid with upward viscous force and downward weight F mg r
Fig: Body falling downward in liquid

Derivation of Stoke’s Law by Dimensional Method

Consider a spherical body of radius ‘r’ falling through a liquid of coefficient of viscosity (η) with terminal velocity ‘v’. According to Stoke’s, viscous force (F) depends upon:

  1. coefficient of viscosity of liquid (η),
  2. terminal velocity of the body (v),
  3. radius of spherical body (r).
In dimensional form:
F ∝ ηavbrc
F = kηavbrc
where k is proportionality constant = 6π.
∴ F = 6πηavbrc   — (i)

Dimensional equation of eqn (i):

[MLT−2] = [ML−1T−1]a[LT−1]b[L]c
[MLT−2] = [MaL−a+b+cT−a−b]

Equating dimensions of both sides:

a = 1
−a − b = −2 ⇒ b = 1
−a + b + c = 1 ⇒ c = 1

Putting a = 1, b = 1, c = 1 in eqn (i):

F = 6πη1v1r1
F = 6πηrv

This is Stoke’s law.

Measurement of Coefficient of Viscosity by Using Stoke’s Law

Let us consider a sphere of radius ‘r’ and density ‘ρ’ falling through a liquid of density ‘σ’ with terminal velocity ‘v’. In this case force acting on a body are:

Sphere falling in liquid showing weight, upthrust and viscous force U F W r v liquid, σ sphere, ρ
Fig: Sphere falling in a liquid

i) Weight of sphere (W) = mg:

W = Vρg
= (4/3)πr3ρg   [in downward direction]

ii) Upthrust (U) = weight of liquid displaced:

U = (4/3)πr3σg

iii) Viscous force:

F = 6πηrv   [in upward direction]

When speed of the body in downward direction increases, the viscous force is also increased and at certain time the upward force acting on a body equals to downward force and body attains equilibrium.

At equilibrium:

F + U = W
6πηrv + (4/3)πr3σg = (4/3)πr3ρg
6πηrv = (4/3)πr3(ρ − σ)g
η = 2r2(ρ − σ)g / (9v)

This is required expression for coefficient of viscosity.

Bernoulli’s Theorem

Bernoulli’s theorem states, “For stream-line flow of an ideal liquid (non-viscous and incompressible), the total energy (the sum of P.E., K.E. & pressure energy) per unit mass remains constant at every cross-section throughout the flow.”
v2/2 + gh + P/ρ = K (constant)
Bernoulli theorem pipe with sections A and B, velocities v1 and v2 and heights h1 and h2 A, A′ B′, B v₁ v₂ h₁ h₂
Fig: Bernoulli’s theorem

Let us consider a pipe AB in which non-viscous and incompressible fluid is flowing. Let a1, P1, v1, h1 and a2, P2, v2, h2 be the area of cross-section, pressure, velocity and height at end A & B respectively. Also let in time Δt the fluid at end A reach to A′ and at end B′ reach to B.

Force acting on the fluid at end A:

F1 = P1a1
W1 = (P1a1)(v1Δt)

Similarly, workdone by fluid at end B′ to B:

W2 = P2a2v2Δt

Thus, net workdone on the fluid from moving A to B:

W = W1 − W2
W = P1a1v1Δt − P2a2v2Δt   — (i)

From equation of continuity:

a1v1 = a2v2 = av (say)
a1v1Δt = a2v2Δt = avΔt = V
W = avΔt(P1 − P2)
W = (P1 − P2)V   — (ii)

Let ‘m’ be the mass of liquid flowing through the pipe in Δt. Then change in K.E.:

ΔK.E. = ½mv22 − ½mv12   — (iii)

Similarly, change in P.E.:

ΔP.E. = mgh2 − mgh1   — (iv)

Total workdone = Total change in energy:

(P1 − P2)V = ½mv22 − ½mv12 + mgh2 − mgh1
P1 − P2 = ½ρv22 − ½ρv12 + ρgh2 − ρgh1
P1/ρ − P2/ρ = v22/2 − v12/2 + gh2 − gh1
P1/ρ + v12/2 + gh1 = P2/ρ + v22/2 + gh2
P/ρ + gh + v2/2 = constant

Equation of Continuity

Steady flow of liquid through a pipe showing areas a1 and a2 and velocities v1 and v2 a₁, ρ₁ a₂, ρ₂ v₁ v₂
Fig: Steady flow of liquid

Let us consider a liquid flowing through pipe of cross-sectional area ‘a1’ and ‘a2’ at left & right end respectively. Also let v1, ρ1 and v2, ρ2 are velocities and density of liquid at respective end.

Volume per second of liquid entering into the pipe at left end:

V1 = a1v1

Again, mass of liquid entering per second at left end:

m1 = ρ1V1
m1 = a1v1ρ1

Similarly, mass of liquid per second at right end:

m2 = a2v2ρ2

If there is no loss of liquid in tube, then:

m1 = m2
a1v1ρ1 = a2v2ρ2

Also, if liquid is incompressible, then ρ1 = ρ2:

a1v1 = a2v2
av = constant

This is called equation of continuity. This equation states that if the area of cross-section of the tube becomes larger then liquid’s speed becomes smaller and vice-versa.

Numericals — Viscosity, Stoke’s Law, Bernoulli & Continuity

Q.5 — Mass of an aeroplane

Calculate mass of an aeroplane with the wings of area 55 m2 flying horizontally. The velocity of air above & below the wings is 155 m/s and 140 m/s respectively.

Source note: the question states wing area 55 m², but the handwritten solution uses A = 50 m². The calculation below preserves the solution exactly as written.
Area of plane (A) = 50 m2
Speed of air over lower wing (v1) = 140 m/s
Speed of air over upper wing (v2) = 155 m/s
ρair = 1.293 kg/m3
Pressure of air over lower & upper wing are P1 & P2 respectively.

Then, by Bernoulli’s equation:

(P1 − P2)/ρ = v22/2 − v12/2   [h1 = h2]
(P1 − P2)A / 1.29 = 50[(155)2 − (140)2]/2 = 110625
F = 110625 × 1.29 = 142706.25 N
mg = 142706.25
m = 142706.25/9.8 = 1.456×104 kg

Q.6 — Terminal velocity of an air bubble

Calculate the magnitude and direction of the terminal velocity of an 1 mm radius air bubble rising in an oil of viscosity 0.20 N m−2s and specific gravity of 0.9 & density of air 1.29 kg/m3.

Source note: the handwritten substitution uses η = 0.02 N s m⁻² (0.2 poise), while the question line appears to show 0.20 N m⁻²s. The calculation below follows the handwritten solution.
r = 1 mm = 10−3 m
ρoil = specific gravity × density of water = 0.9×103 kg/m3
ρair = 1.29 kg/m3
η = 0.02 N s m−2 = 0.2 poise
For air bubble, from Stoke’s law:
vt = 2r2air − ρoil)g/(9η)
= 2(10−3)2(1.29 − 900)×10 / [9(0.02)]
= −9.99×10−3 m/s
Magnitude = 0.0099 m/s; direction = upward

Q.7 — Eight raindrops coalesce

Eight/three/two spherical raindrops of equal size are falling vertically through air with terminal velocity 0.15 m/s. If they coalesce what would be the terminal velocity?

For 8 small drops of radius r and one big drop of radius R:
8 × (4/3)πr3 = (4/3)πR3
R3 = (2r)3
R = 2r

By Stoke’s relation v ∝ r²:

v1/v2 = r2/R2
v2 = v1R2/r2
= 0.15(2r)2/r2
v2 = 0.6 m/s

Q.8 — Steel ball in castor oil

Castor oil at 20°C has a coefficient of viscosity 2.42 N m−2s and density 940 kg/m3. Calculate the terminal velocity of the steel ball of radius 2.00 mm falling under gravity in the oil, taking density of steel as 7800 kgm−3.

T = 20°C
η = 2.42 N s m−2
σ = 940 kg/m3
ρ = 7800 kg/m3
r = 2 mm = 2×10−3 m
vt = 2r2(ρ−σ)g/(9η)
= 2(2×10−3)2(7800−940)×10 / [9(2.42)]
vt = 0.025 m/s

Q.9 — Water through non-uniform pipe

Water flows steadily through a horizontal pipe of non-uniform cross-section. If the pressure of the water is 4×104 N m−2 at a point where the velocity of flow is 2 m s−1 & cross-section is 0.02 m2, what is the pressure at the point where cross section reduces to 0.01 m2?

Horizontal non-uniform pipe for continuity and Bernoulli numerical A₁ = 0.02 m² A₂ = 0.01 m² v₁ = 2 v₂ = ?
Non-uniform horizontal pipe
P1 = 4×104 N/m2
v1 = 2 m/s
A1 = 0.02 m2
A2 = 0.01 m2
ρwater = 1000 kg/m3

From equation of continuity:

A1v1 = A2v2
(0.02)2 = (0.01)v2
v2 = 4 m/s

Again, from Bernoulli’s principle (where h1 = h2):

P1/ρ + v12/2 = P2/ρ + v22/2
4×104/1000 + (2)2/2 = P2/1000 + (4)2/2
P2 = 34000 N/m2

Q.10 — Glass ball falling through glycerol

What is the terminal velocity of the glass ball falling through a tall jar containing glycerol? The densities of the glass ball and glycerol are 8.5 g/cc and 1.32 g/cc respectively & viscosity of the glycerol is 0.85 poise & radius of the ball is 2 mm.

ρ = 8.5 g/cc = 8.5×103 kg/m3
σ = 1.32 g/cc = 1.32×103 kg/m3
η = 0.85 poise = 0.085 N s m−2
r = 2 mm = 2×10−3 m
vt = 2r2(ρ−σ)g/(9η)
= 2(2×10−3)2(8.5−1.32)×103×10 / [9(0.085)]
vt = 0.75 m/s
The last handwritten sentence calls it a “steel ball”, although the question asks for a glass ball. The numerical value above is preserved from the source.

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