Buy BOOKS at Discounted Price

Surface Areas And Volumes

Class 10th Mathematics KC Sinha Solution
Exercise 14.1
  1. Three cubes each of side 5 cm are joined end to end to form a cuboid. Find the…
  2. Cardboard boxes of two different sizes are made. The bigger has dimensions…
  3. The length of cold storage is double its breadth. Its height is 3 metres. The…
  4. Find: (i) the lateral surface, (ii) the whole surface, and (iii) the volume of…
  5. The radius and height of a right circular cone are in the ratio of 5:12. If its…
  6. A cylinder, whose height is two-thirds of its diameter, has the same volume as…
Exercise 14.2
  1. A toy is in the form a cone mounted on a hemisphere of diameter 7 cm. The total…
  2. A solid is in the form of a right circular cone mounted on a hemisphere. The…
  3. A medicine capsule is in the shape of a cylinder with two hemispheres stuck to…
  4. A room in the form of a cylinder, surmounted by a hemispherical vaulted dome,…
  5. The interior of a building is in the form of a cylinder of diameter 4.3 m and…
  6. A tent of height 77 dm is in the form of a right circular cylinder of diameter…
  7. A tent of height 3.3 m is in the form of a right circular cylinder of diameter…
  8. A medicine capsule as shown in the given figure is in the shape of a cylinder…
  9. A vessel is in the form of a hollow hemisphere mounted by a hollow cylinder.…
  10. From a circular cylinder of base diameter 10cm and height 12cm, a conical…
  11. An ice-cream cone consists of a right circular cone of height 14cm and…
  12. A student was asked to make a model in his workshop, which shaped like a…
  13. A decorative block as shown in the given figure is made of a cube and a…
  14. A godown building is in the form as shown in the adjoining figure. The…
  15. A solid iron pole is having a cylindrical portion 110 cm high and of base…
  16. A wooden toy rocket is in the shape of a cone mounted on a cylinder, as shown…
  17. The inner diameter of glass is 7cm, and it has a raise portion in the bottom…
Exercise 14.3
  1. Two cylindrical vessels are filled with oil. The radius of one vessel is 15 cm…
  2. A metal cube of edge 12 cm is melted and formed into three smaller cubes. If…
  3. A hemisphere of lead of radius 8 cm is cast into a right circular cone of base…
  4. A solid sphere of radius 3 cm is melted and then recast into small spherical…
  5. A solid metal cone with radius of base 12 cm and height 24 cm is melted to form…
  6. A solid metallic sphere of diameter 21 cm is melted and recast into a number of…
  7. Spherical ball of diameter 21cm is melted and recasted into cubes, each of side…
  8. The internal and external diameters of a hollow hemispherical shell are 6cm and…
  9. A solid sphere of radius 6cm is melted into a hollow cylinder of uniform…
  10. The diameter of a copper sphere is 6cm. The sphere is melted and is drawn into…
  11. A cylindrical container is filled with ice-cream. Its diameter is 12cm and…
Exercise 14.4
  1. The radii of the ends of a frustum of a cone 45 cm high are 28cm and 7cm. Find…
  2. A drinking glass is in the shape of a frustum of a cone of height 14cm. The…
  3. The radii of the circular ends of a solid frustum of a cone are 33 cm and 27cm,…
  4. The perimeters of the ends of the frustum of a cone are 96 cm and 68 cm. If the…
  5. A friction clutch in the form of the frustum of a cone, the diameters of the…
  6. A bucket is in the form of a frustum of a cone. Its depth is 15cm and the…
  7. An open metal bucket is in the shape of a frustum of a cone, mounted on a…
  8. A bucket made up of a metal sheet is in the form of frustum of a cone. Its…
  9. A tent is made in the form of a conic frustum, surmounted by a cone. The…
  10. An oil funnel of tin sheet consists of a cylindrical portion 8 cm along…

Exercise 14.1
Question 1.

Three cubes each of side 5 cm are joined end to end to form a cuboid. Find the surface area of the resulting cuboid.


Answer:



Length of each side of cube = 5cm


According to question,


Three cubes are joined end to end to form a cuboid


So, the length of the resulting cuboid = 5 + 5 + 5 = 15cm


But, the breadth and height will remain the same


So, Breadth = 5cm


and height = 5cm


Surface Area of the Cuboid


= 2(Length × Breadth + Breadth × Height + Height × Length)


= 2(lb + bh + hl)


= 2(15×5 + 5×5 + 5×15)


= 2(75 + 25 + 75)


= 2(175)


= 350cm2


Hence, the surface area of cuboid is 350cm2.



Question 2.

Cardboard boxes of two different sizes are made. The bigger has dimensions 20cm, 15cm and 5cm and the smaller dimensions 16cm, 12cm and 4cm. 5% of the total surface area is required extra for all overlaps. If the cost of the cardboard is Rs. 20 for one square metre, find the cost of the cardboard for supplying 200 boxes of each kind.


Answer:


For bigger box:


Length of the bigger box = 20cm


Breadth of the bigger box = 15cm


Height of the bigger box = 5cm


So, Total Surface Area of bigger box = 2(lb + bh + hl)


= 2(20×15 + 15×5 + 5×20)


= 2(300 + 75 + 100)


= 2(475)


= 950cm2


For Smaller box:


Length of the smaller box = 16cm


Breadth of the smaller box = 12cm


Height of the smaller box = 4cm


So, Total Surface Area of bigger box = 2(lb + bh + hl)


= 2(16×12 + 12×4 + 4×16)


= 2(192 + 48 + 64)


= 2(304)


= 608cm2


Total surface area of 200 boxes of each type = 200(950 +608)


= 200 × 1558


= 311600cm2


Extra Area Required = 5% of (950 + 608)



= 15580cm2


So, Total cardboard Required = 311600 + 15580


= 327180cm2



Cost of Cardboard for 1m2 = Rs 20


Cost of Cardboard for 327.18 m2 = 20 × 327.18


= Rs 654.36


Hence, the cost of the cardboard for supplying 200 boxes of each kind is Rs 654.36.



Question 3.

The length of cold storage is double its breadth. Its height is 3 metres. The area of its four walls (including doors) is 108 m2. Find its volume.


Answer:

Given: Area = 108m2, Height = 3m


and Length of a cold storage is double its breadth


So, Let breadth of a cold storage = x


∴ length of the cold storage = 2x


Area of four walls = 108m2


∴ 2(l + b)× h = 108


⇒ 2 (x + 2x)×3 = 108


⇒ 6(3x) = 108



⇒ x = 6


∴ Breadth of a cold storage = 6m


and Length of a cold storage = 2×6 = 12m


Hence, Volume of the cold storage = l × b × h


= 12 × 6 × 3


= 216m3



Question 4.

Find:

(i) the lateral surface,

(ii) the whole surface, and

(iii) the volume of a right circular cylinder whose height is 13.5 cm and radius of the base 7cm


Answer:

Given: Radius of base, r = 7cm


Height , h = 13.5cm


(i) Lateral Surface Area of right circular cylinder = 2πrh



= 2×22×13.5


= 594cm2


(ii) Total Surface Area of cylinder = 2πrh + 2πr2


= 2πr(h + r)



= 2×22×20.5


= 902cm2


(iii) Volume of cylinder = πr2h



= 22×7×13.5


= 2079cm3



Question 5.

The radius and height of a right circular cone are in the ratio of 5:12. If its volume is 314 cm3, find its slant height.

[Take π=3.14]


Answer:

Given: Volume of a right circular cone = 314cm3


and r : h = 5 : 12


Let r = 5x and h = 12x


The volume of a right circular cone





⇒ x3 = 1


Hence, x = 1


∴ r = 5x = 5×1 = 5cm


and h = 12x = 12×1 = 12cm


Now, we have to find the slant height, l


Slant height, l = √(r2 + h2)


= √{(5)2 + (12)2}


= √25+144


= √169


= ±13


= 13cm


[taking positive root, because slant height can’t be negative]


∴ Slant Height = 13cm



Question 6.

A cylinder, whose height is two-thirds of its diameter, has the same volume as a sphere of radius 4 cm. Calculate the radius of the base of the cylinder.


Answer:

Let height of a cylinder = h


and diameter = d


According to question,





Given: Radius of Sphere, R = 4cm


Volume of Cylinder = Volume of Sphere (Given)




⇒ r3 = R3


⇒ r3 = 43


⇒ r = 4cm


Hence, Radius of base of the cylinder is 4cm.




Exercise 14.2
Question 1.

A toy is in the form a cone mounted on a hemisphere of diameter 7 cm. The total height of the toy is 14.5 cm. Find the volume of the toy.


Answer:


Given: Diameter of a hemisphere = 7cm


So,


Height of the cone, h = 143-.5 – 3.5 = 11cm


The volume of a toy = Volume of cone + Volume of a hemisphere






= 231cm3



Question 2.

A solid is in the form of a right circular cone mounted on a hemisphere. The radius of the hemisphere is 2.1 cm, and the height of the cone is 4 cm. The solid is place in a cylindrical tub full of water in such a way that the whole solid is submerged in water left in the tub.


Answer:

Original volume of water in the cylindrical tub

= Volume of Cylinder


= πr2h



= 22×25×1.4


= 770cm3


Given that Radius of hemisphere, R = 2.1cm


and height of cone, h = 4cm


Volume of a solid = Volume of cone + Volume of hemisphere






= 37.884cm3


∴ Volume of water displaced (removed) = 37.884cm3


Hence, the required volume of the water left in the cylindrical tub = 770 – 37.884


= 732.116 cm3



Question 3.

A medicine capsule is in the shape of a cylinder with two hemispheres stuck to each of its ends. The length of the entire capsule is 14 mm, and the diameter of the capsule is 5 mm. Find its surface area.


Answer:


Given:


Diameter of cylinder = 5mm


The radius of cylinder


Height of cylinder = 14 – 5 = 9mm


Here, Diameter of hemisphere =. 5mm


So,


The total area of the capsule = CSA of cylinder + CSA of 2 hemispheres


= 2πrh + 2×2πr2




=220 mm2



Question 4.

A room in the form of a cylinder, surmounted by a hemispherical vaulted dome, contains m3 of air and the internal diameter of the building is equal to the height of the crown of the vault above the floor. Find the height


Answer:


Let r be the radius of hemisphere and cylinder


and height of the cylinder = h


Given:


∴ Diameter of the building = 2r


Height of the building (H) = diameter of the building


∴ Height of the cylinder + Radius of hemispherical dome = 2r


⇒ h + r = 2r


⇒ h = 2r – r


⇒ h = r


Volume of air inside the building = Volume of hemispherical portion


+ Volume of cylindrical portion








⇒ r3 = 8


⇒ r = 2


⇒ Height of building = 2r = 2×2 = 4m


Hence, the total height of the building is 4m.



Question 5.

The interior of a building is in the form of a cylinder of diameter 4.3 m and height 3.8 m surmounted by a cone whose vertical angle is a right angle. Find the area of the surface and the volume of the building. [Take π=3.14]


Answer:


The diameter of a cylindrical portion BCDE of building = 4.3m


∴ The radius of a cylindrical portion


Height = 3.8m


Lateral Surface Area of Cylindrical Portion BCDE = 2πrh



= 51.3543 m2


Let AB be the slant height of the conical portion of the building = l = AB = AC


Now, the Lateral surface of conical portion = πrl



= 20.5417 m2


So,


The total surface area of the building = Surface area of cylindrical portion + Surface area of the conical portion


= 51.3543 + 20.5417


= 71.8960


= 71.90 m2 (approx.)


Now, In right ΔBAC,


BC2 = AB2 + AC2


⇒ BC2 = l2 + l2


⇒ (4.3)2 = 2l2


⇒ 18.49 = 2l2



⇒ l2 = 9.245


⇒ l = 3.04 m


Here, r is the radius of the cone and l is the slant height of the cone


⇒ l2 = h2 + r2


⇒ 9.245 = h2 + (2.15)2


⇒ 9.245 = h2 + 4.6225


⇒ h2 = 9.245 – 4.6225


⇒ h2 = 4.6225


⇒ h = 2.15 m


Now, Volume of the conical portion



= 10.4116 m3


Volume of cylindrical portion BCDE = πr2h



= 55.2059 m3


So,


The total volume of the building = Volume of the cylindrical portion


+ Volume of the conical portion


= 55.2059 + 10.4116


= 65.6175 m3


= 65.62 m3 (approx.)



Question 6.

A tent of height 77 dm is in the form of a right circular cylinder of diameter 36m and height 44 dm surmounted by a right circular cone. Find the cost of the canvas at Rs 3.50 per m2.



Answer:


Given: Height of the tent = 77dm



The height of the conical portion = 44dm



∴ The height of the cylindrical portion


= Height of the tent – Height of the conical portion


= 7.7 – 4.4


= 3.3m


Given Diameter of the cylinder, d = 36m



CSA of cylindrical portion = 2πrh



= 497.828


= 497.83 m2 (approx.)


Firstly, we have to find the slant height (l) of the conical portion


⇒ l2 = h2 + r2


⇒ l2 = (3.3)2 + (18)2


⇒ l2 = 10.89 + 324


⇒ l2 = 334.89


⇒ l = √334.89


⇒ l = 18.3m


∴ CSA of the conical portion = πrl



= 1035.257


= 1035.26 m2 (approx.)


So,


Total Surface Area of the tent = Surface area of conical portion + surface Area of cylindrical portion


= 1035.26 + 497.83


= 1533.09 m2


∴ Canvas required to make the tent = 1533.09 m2


Cost of 1m2 canvas = Rs 3.50


Cost of 1533.09m2 canvas = Rs 3.50 × 1533.09


= Rs 5365.815


= Rs 5365.82



Question 7.

A tent of height 3.3 m is in the form of a right circular cylinder of diameter 12m and height 2.2m, surmounted by a right circular cone of the same diameter. Find the cost of the canvas of the tent at the rate of Rs. 500 per m2.


Answer:


Given: Height of the tent = 3.3 m


The height of the cylindrical portion = 2.2 m


∴ The height of the conical portion


= Height of the tent – Height of the cylindrical portion


= 3.3 – 2.2


= 1.1m


Given Diameter of the cylinder, d = 12m



CSA of cylindrical portion = 2πrh



= 82.971 m2


Firstly, we have to find the slant height (l) of the conical portion


⇒ l2 = h2 + r2


⇒ l2 = (1.1)2 + (6)2


⇒ l2 = 1.21 + 36


⇒ l2 = 37.21


⇒ l = √37.21


⇒ l = 6.1m


∴ CSA of the conical portion = πrl



= 115.029 m2


So,


Total Surface Area of the tent = Surface area of conical portion + surface Area of cylindrical portion


= 115.029 + 82.971


= 198 m2


∴ Canvas required to make the tent = 198 m2


Cost of 1m2 canvas = Rs 500


Cost of 198 m2 canvas = Rs 500 × 198


= Rs 99000



Question 8.

A medicine capsule as shown in the given figure is in the shape of a cylinder with two hemispheres stuck to each of its ends. The length of the entire capsule is 12 mm, and the diameter of the capsule is 5 mm. Find its surface area.


Answer:


Given:


Diameter of cylinder = 5mm



Height of cylinder = 12 – 5 = 7mm


Here, Diameter of hemisphere = 5mm


So,Radius of Hemisphere =


Total area of the capsule = CSA of cylinder + CSA of 2 hemispheres


= 2πrh + 2×2πr2







Question 9.

A vessel is in the form of a hollow hemisphere mounted by a hollow cylinder. The diameter of the hemisphere is 14 cm, and the total height of the vessel is 13cm. Find the inner surface area of the vessel.



Answer:


Given: Diameter of the hemisphere = 14cm


∴ Radius of the hemisphere


Curved Surface Area of Hemisphere = 2πr2



= 308cm2


Now,


Radius of cylinder = radius of hemisphere = 7cm


Height of cylinder = Total height – Radius of hemisphere


= 13 – 7


= 6cm


So, CSA of cylinder = 2πrh



= 264cm2


The inner Surface area of the vessel


= CSA of hemisphere + CSA of cylinder


= 308 + 264


= 572cm2


Hence, Inner surface area of vessel is 572cm2



Question 10.

From a circular cylinder of base diameter 10cm and height 12cm, a conical cavity with the same base and height is carved out. Find the volume of the remaining solid.


Answer:

Given that Diameter of circular cylinder = 10cm

∴ Radius of the cylinder


and Height of the cylinder = 12cm


So,


Volume of the cylinder = πr2h



= 942.857 cm3




= 314.285 cm3


Remaining Volume = Volume of the cylinder – Volume of cone


= 942.857 – 314.285


= 628.572 cm3



Question 11.

An ice-cream cone consists of a right circular cone of height 14cm and diameter of the circular top is 5 cm. It has hemisphere on the top with the same diameter as of circular top. Find the volume of ice-cream in the cone.


Answer:

Given: Height of the cone = 14cm


and diameter of the circular top = 5cm



Now,




= 91.6666cm3


Now,




= 32.738 cm3


So,


The volume of ice cream in the cone


= Volume of cone + Volume of a hemisphere


= 91.6666 + 32.738


= 124.404cm3



Question 12.

A student was asked to make a model in his workshop, which shaped like a cylinder with two cones attached at its two ends, using a thin aluminium sheet. The diameter of the model is 3 cm and its length is 10 cm. If each cone has a height of 2cm, find the volume of air contained in the model. (Consider the outer and inner dimensions of the model to be nearly the same).


Answer:


Given that diameter of cylinder = 3cm


So,


Given: Height of cone = 2cm


and


Height of the cylinder + height of cone + height of cone = 10cm


Height of the cylinder + 2 + 2 = 10


Height of the cylinder = 6cm


So,


Volume of the cylinder = πr2h



= 42.428 cm3


= 42.43 cm3 (approx.)




= 4.71 cm3


Volume of 1st cone = Volume of 2nd cone = 4.71cm3


So, Total volume of the model


= Volume of cylinder + volume of 1st cone + volume of 2nd cone


= 42.43 + 4.71 + 4.71


= 51.85cm3



Question 13.

A decorative block as shown in the given figure is made of a cube and a hemisphere. The edge of the cube is 10cm, and the radius of the hemisphere attached on the top is 3.5. Find the cost of painting the block at the rate of 50 paise per sq.cm.



Answer:


Given that side of a cube = 10cm


Total Surface Area of cube = 6(side)2


= 6 × 10 × 10


= 600cm2


Now, Radius of hemisphere = 3.5cm


Curved Surface Area of hemisphere = 2πr2


= 77cm2


Base area of hemisphere = Area of circle = πr2



= 38.5cm2


Therefore,


Total Surface Area of block = Total Surface area of a cube + CSA of the hemisphere - Base area of a hemisphere


= 600 + 77 – 38.5


= 638.5cm2


Cost of painting the block of 1cm2 = Rs 0.50


Cost of painting the block of 638.5cm2 = Rs 0.50 × 638.5


= Rs 319.25



Question 14.

A godown building is in the form as shown in the adjoining figure. The vertical cross-section parallel to the width side of the building is a rectangle of size 7 m x 3 m mounted by a semicircle of radius 3.5 m. The inner measurement of the cuboidal portion are 10 m x 7 m x 3 m. Find the

(i) the volume of the godown, and

(ii) the total internal surface area excluding the floor.



Answer:

(i) Volume of godown = Volume of cuboid +volume of cylinder


Volume of cuboid = l × b × h = 10 × 7 × 3 = 210 m3


Volume of cylinder = πr2h = = 385 m3


∴ Volume of godown


= 210 + 192.5


= 402.5 cm3


(ii) Internal Surface Area of a godown excluding the floor


the total surface area of the cylinder




= 6 × 17 + 22 × 0.5 × 13.5


= 102 + 148.5


= 250.5 cm2


Hence, internal surface area of godown excluding floor is 250.5cm2



Question 15.

A solid iron pole is having a cylindrical portion 110 cm high and of base diameter, 12 cm is surmounted by a cone 9 cm high. Find the mass of the pole, given that mass of 1 cm3 of iron is 8 g.


Answer:


Given: Diameter of cone = 12cm


So, Radius of cone = 6cm


and Height of cone = 9cm




= 339.428 cm3


Radius of the cylinder, R = Radius of cone = 6cm


Height of the cylinder, H = 110cm


Volume of cylinder = πR2H



= 12445.714cm3


Hence, the volume of the pole


= Volume of conical part + Volume of the cylindrical part


= 339.43 + 12445.714


= 12785.142 cm3


Required mass of pole = 8 × 12785.142


= 102281.13 gm


= 102.281kg



Question 16.

A wooden toy rocket is in the shape of a cone mounted on a cylinder, as shown in the given figure. The height of the entire rocket is 26 cm, while the height of the conical part is 6cm. The base of the conical portion has a diameter of 5cm, while the base diameter of the cylindrical portion is 3cm. If the conical portion is to be painted orange and the cylindrical portion yellow, find the area of the rocket painted with each of these colours. [Take π=3.14]



Answer:


The area to be painted orange


= CSA of cone + Base area of Cone – Base area of the cylinder


Curved Surface Area of Cone


Given that diameter of conical portion = 5cm


The radius of conical portion


Height of the conical part = h = 6cm


We need to find the ‘l’ first


We know that


l2 = h2 + r2


⇒ l2 = 62 + (2.5)2


⇒ l2 = 36 + (6.25)


⇒ l2 = 42.25


⇒ l = √42.25


⇒ l = 6.5cm


So, CSA of conical portion = πrl


=3.14×2.5×6.5


= 51.025 cm2


Base area of the cone = πr2


=3.14×2.5×2.5


=19.625 cm2


Diameter of the cylinder = 3cm


So, Radius of the cylinder = 1.5cm


Base area of the cylinder = π(r’)2


=3.14×1.5×1.5


=7.065 cm2


So, the area to be painted orange = 51.025 + 19.625 – 7.065


= 63.585 cm2


Now, the area to be painted yellow


= CSA of the cylinder + Area of one bottom base of the cylinder


= 2πr’h’ + π(r’)2


= 2 × 3.14 × 1.5 × 20 + 7.065


= 188.4 + 7.065


= 195.465 cm2



Question 17.

The inner diameter of glass is 7cm, and it has a raise portion in the bottom in the shape of a hemisphere as shown in the figure. If the height of the glass is 16 cm, find the apparent capacity and the actual capacity of the glass.



Answer:

Given the inner diameter of the glass = 7cm


So, the radius of the glass =


Height of the glass = 16cm


The volume of the cylindrical glass = πr2h



= 616cm3


Now, radius of the hemisphere = Radius of the cylinder =r = 3.5cm


Volume of the hemisphere



= 89.83 cm3


Now,


Apparent capacity of the glass = Volume of cylinder = 616cm3


The actual capacity of the glass


= Total volume of cylinder – Volume of hemisphere


= 616 – 89.83


= 526.17cm3


Hence,


Apparent Capacity of the glass = 616cm3


and actual capacity of the glass = 526.17cm3




Exercise 14.3
Question 1.

Two cylindrical vessels are filled with oil. The radius of one vessel is 15 cm and its height is 25 cm. The radius and height of the other vessel are 10cm and 18 cm respectively. Find the radius of a cylindrical vessel 30 cm in height, which will just contain the oil of two given vessels.


Answer:

Radius of 1st cylindrical vessel = 15cm


and height = 25cm


Radius of 2nd cylindrical vessel = 10cm


and height = 18cm


So, Volume of 1st cylindrical vessel = πr2h


= π × (15)2 × 25


= 5625π cm3


Volume of 2nd cylindrical vessel = π(r’)2h’


= π × (10)2 × 18


= 1800π cm3


Height of the third vessel = 30cm


and let its radius be R


So,


Volume of third cylindrical vessel = πR2H


= πR2×30


= 30πR2


Volume of 1st cylindrical vessel + Vol. of 2nd Cylindrical vessel


= Volume of the third cylindrical Vessel


⇒ 5625π + 1800π = 30πR2


⇒ 7425π = 30πR2



⇒ R2 = 247.5


⇒ R = 15.73cm


Hence, radius of the required cylinder is 15.73cm



Question 2.

A metal cube of edge 12 cm is melted and formed into three smaller cubes. If the edges of two smaller cubes are 6 cm and 8 cm, find the edge of the third smaller cube (Assume that there is no loss of metal during melting)


Answer:

Let the edge of the third smaller cube be x cm.


Three small cubes are formed by melting the cube of edge 12 cm.


Edges of two small cubes are 6 cm and 8 cm.


Now, volume of a cube = (side)3


Volume of the big cube = sum of the volumes of the three small cubes


⇒ (12)3 = (6)3 + (8)3 + (x)


⇒ 1728 = 216 + 512 + x3


⇒ 1728 = 728 + x3


⇒ x3 = 1728 – 728


⇒ x3 = 1000


⇒ x = 10 cm


Therefore, the edge of the third cube is 10 cm



Question 3.

A hemisphere of lead of radius 8 cm is cast into a right circular cone of base radius 6 cm. Determine the height of the cone, correct to two places of decimals.


Answer:

Radius of hemisphere = 8cm


and Radius of right circular cone = 6cm


According to question,


Volume of the hemisphere = Volume of cone





⇒ h = 28.44cm



Question 4.

A solid sphere of radius 3 cm is melted and then recast into small spherical balls each of diameter 0.6 cm. Find the number of small balls thus obtained.


Answer:

Given that Radius of bigger sphere (R) = 3cm


and diameter of smaller spherical balls = 0.6cm


So, radius of small spherical balls (r) = 0.3cm


Let the number of small balls = n


According to the question,


n × volume of small balls = Volume of bigger sphere



⇒ nr3 = R3





⇒ n = (10)3


⇒ n = 1000


Hence, the number of small balls = 1000



Question 5.

A solid metal cone with radius of base 12 cm and height 24 cm is melted to form solid spherical balls of diameter 6 cm each. Find the number of balls thus formed.


Answer:

Given that Radius of cone = 12cm


Height of the cone = 24cm


So,




= 1152π cm3


It is also given that diameter of each spherical balls = 6cm


so, Radius of each ball = 3cm




= 36π cm3


Total number of balls formed by melting the cone




= 32


Hence, 32 balls are formed by melting the cone.



Question 6.

A solid metallic sphere of diameter 21 cm is melted and recast into a number of smaller cones, each of diameter 3.5 cm and height 3cm. Find the number of cones so formed.


Answer:

Let the number of cones formed be n


Diameter of a metallic sphere = 21cm


So,



=


= 11 × 21 × 21


= 4851cm3


Now, Diameter of cone = 3.5cm


So,


and height = 3cm



=


cm3


According to question,


n × volume of cone = volume of sphere




⇒ n = 504


Hence, the number of cones formed = 504



Question 7.

Spherical ball of diameter 21cm is melted and recasted into cubes, each of side 1 cm. Find the number of cubes thus formed. [Use π=22/7]


Answer:

Diameter of sphere = 21 cm


So,



=


= 11 × 21 × 21


= 4851cm3


Volume of cube = a3 = 13


Let number of cubes formed be n, then


Volume of sphere = n × (volume of cube)


⇒ 4851 =n×1


⇒ n = 4851


Hence, number of cubes is 4851.



Question 8.

The internal and external diameters of a hollow hemispherical shell are 6cm and 10cm respectively. It is melted and recast into a solid cone of base diameter 14cm. Find the height of the cone so formed.


Answer:

Given:


Internal Diameter of a hollow hemispherical shell = 6cm


So, internal radius, (r) = 3cm


External Diameter of a hollow hemispherical shell = 10cm


So, external radius (R) = 5cm





Given that diameter of cone =14cm


So, radius = 7cm




According to the question.


Volume of a shell = Volume of a cone




⇒ h = 4cm



Question 9.

A solid sphere of radius 6cm is melted into a hollow cylinder of uniform thickness. If the external radius of the base of the cylinder is 5cm and its height is 32 cm, find the uniform thickness of the cylinder.


Answer:

Let inner radius of the cylinder be x cm


Given that radius of sphere = 6cm



=


= 4 × π × 2 × 36 cm3


It is also given that external radius of base of cylinder (R) = 5cm


and height = 32cm


∴ Volume of hollow cylinder = πh[R2 – r2]


= π × 32 × [(5)2 – x2]


= 32π [25 – x2]


Solid sphere is melted and casted into hollow cylinder


∴ Volume of sphere = Volume of hollow cylinder


⇒ 4 × π × 2 × 36 = 32π [25 – x2]



⇒ 9 – 25 = -x2


⇒ x2 = 16


⇒ x = ±4


⇒ x = 4cm


Hence, the inner radius of the cylinder is 4cm


So, the thickness of the cylinder = external radius – inner radius


= 5 – 4


= 1cm



Question 10.

The diameter of a copper sphere is 6cm. The sphere is melted and is drawn into a long wire of uniform circular cross-section. If the length of the wire is 36cm, find its radius [Take π=2.14]


Answer:

Diameter of a copper sphere = 6 cm


So, Radius of copper sphere = 3cm



=


= 36π cm3


Length of wire = 36cm


Let the radius of wire be R cm


Volume of wire = πR2h


=πR2×36


But the volume of wire = volume of sphere


⇒ 36πR2 = 36π


⇒ R2 = 1


⇒ R = 1cm [taking positive root, because radius can’t be negative]


Hence, the radius of wire is 1cm



Question 11.

A cylindrical container is filled with ice-cream. Its diameter is 12cm and height is 15cm. The whole ice-cream is distributed among 10 children in equal cones having hemispherical tops. If the height of the conical portion is twice the diameter of its base, find the diameter of the ice-cream cone.


Answer:

Let the radius of the base of conical ice cream = x cm


Then, height of the conical ice cream = 2 × diameter


= 2 × (2x)


= 4x cm


Volume of ice- cream cone


= Volume of conical portion + Volume of hemispherical portion





= 2πx3 cm3


Now,


Diameter of cylindrical container = 12cm


So, radius = 6cm


and height = 15cm


∴ Volume of cylindrical container = πr2h


= π × (6)2 × (15)


= 540π cm3





⇒ x3 = 27


⇒ x = 3


So, the radius of the base of conical ice cream is 3cm


Hence, the diameter of the base of conical ice-cream = 2×3 = 6cm




Exercise 14.4
Question 1.

The radii of the ends of a frustum of a cone 45 cm high are 28cm and 7cm. Find its volume, the curved surface area. [Take π=22/7]


Answer:


Frustum = difference of two right circular cones OAB and OCD


Let the height of the cone OAB be h1 and its slant height l1


i.e. OA = OB = l1 and OP = h1.


Let h2 be the height of cone OCD and l2 its slant height


i.e. OC = OD = l2 and OQ = h2


We have , r1 = 28cm and r2 = 7cm


and height of frustum (h) = 45cm


Also,


h1 = 45 + h2 …(i)


Now, we first need to determine the h1 and h2


∵ ΔOPB and OQD are similar, we have



⇒ h1 = 4h2 …(ii)


From (i) and (ii), we get


4h2 = 45 + h2


⇒ 3h2 = 45


⇒ h2 = 15cm


Putting the value of h2 in eq. (ii), we get


h1 = 4×15 = 60cm


So, h1 = 60cm and h2 = 15cm


Now, the volume of frustum = Vol. of cone OAB – Vol. of cone OCD







= 5 × 22 × 441


= 48510 cm3


Now, we first have to find the slant height l1 and l2


l1 = √{(28)2+(60)2}


⇒ l1 = √{(4×7)2 + (4×15)2


⇒ l1 = 4√{(7)2 + (15)2}


⇒ l1 = 4√49 + 225


⇒ l1 = 4√(274)


⇒ l1 = 4 × 16.55


⇒ l1 = 66.20cm


and l2 = √{(7)2 + (15)2}


⇒ l2 = √49 + 225


⇒ l2 = √(274)


⇒ l2 = 16.55 cm


So, CSA of frustum = CSA of cone OAB – CSA of cone OCD


= πr1l1 – πr2l2



= 22 [4×66.20 – 16.55]


= 22 × 248.25


= 5461.5cm2



Question 2.

A drinking glass is in the shape of a frustum of a cone of height 14cm. The diameters of its two circular ends are 4cm and 2cm. Find the capacity of the glass.


Answer:


Given: Height of frustum = 14cm


Diameter of 1st circular end = 4cm


So, radius (R) = 2cm


Diameter of 2nd circular end = 2cm


So, radius (r) = 1cm


Capacity of the glass = Volume of frustum






= 102.66 cm3


Hence, the volume of glass = 102.66cm3



Question 3.

The radii of the circular ends of a solid frustum of a cone are 33 cm and 27cm, and its slant height is 10cm. Find its capacity and total surface area. [Take π=22/7]


Answer:


Greater radius = R = 33 cm


Smaller radius = r = 27 cm


Slant height = l = 10 cm


Total Surface area of the frustum = πR2 + πr2 + πl(R+r)


= π[(33)2 + (27)2+ 10×(33+27)]


= π[1089 + 729 + 600]



= 7599.428 cm2


= 7599.43 cm2


We first need to find the height, h


l2 = (R – r)2 + h2


⇒ (10)2 = (33 – 27)2 + h2


⇒ 100 = (6)2 + h2


⇒ 100 = 36 + h2


⇒ h2 = 100 – 36


⇒ h2 = 64


⇒ h = ±8


⇒ h = 8cm


Now, Capacity of a solid frustum of a cone = Volume of frustum






= 22704 cm3



Question 4.

The perimeters of the ends of the frustum of a cone are 96 cm and 68 cm. If the height of the frustum be 20 cm, find its radii, slant height, volume and total surface. [Take π=22/7]


Answer:


Let radii of the circular ends of the frustum are R and r respectively.


Given that Perimeter of one end = 96cm


⇒ 2πR = 96




⇒ R = 15.27 cm


Perimeter of other end = 68cm


⇒ 2πr = 68




⇒ r = 10.82 cm


It is given that height of the frustum = 20cm


So, Slant height, l = √{h2 + (R – r)2}


= √{(20)2 +(15.27 – 10.82)2


= √400 + (4.45)2


= √400 + 19.80


= √419.80


= 20.49 cm


Now,






= 10800.25 cm3


Total Surface Area of the frustum = πR2 + πr2 + πl(R+r)


= π[(15.27)2 + (10.82)2+ 20.49×(15.27+10.82)]


= π[233.1729 + 117.0724 + 534.5841]



= 2780.89 cm2



Question 5.

A friction clutch in the form of the frustum of a cone, the diameters of the ends being 8cm, and 10 cm and length 8 cm. Find its bearing surface and its volume. [Take π=3.14]


Answer:


We have,


Diameter of one end = 10cm


So, radius (R) = 5cm


Diameter of other end = 8cm


So, radius (r) = 4cm


and slant height, l = 8cm


So, height, h = √{l2 – (R – r)2}


= √{(8)2 – (5 – 4)2


= √64 – (1)2


= √64 – 1


= √63


= 7.937 cm


Bearing surface of the clutch = CSA of the frustum


= πl (R + r)


= 3.14 × 8 (5 + 4)


= 25.12 (9)


= 226.08 cm2






= 506.75 cm3


Hence, Volume of frustum = 506.75 cm3



Question 6.

A bucket is in the form of a frustum of a cone. Its depth is 15cm and the diameters of the top and the bottom are 56 cm and 42 cm respectively. Find how many litres of water can the bucket hold. [Take π=22/7]


Answer:

Greater diameter of the frustum = 56 cm

So, radius of the frustum = R = 28 cm


Smaller diameter of the frustum = 42 cm


So, Radius of the smaller end of the frustum = r = 21 cm


and Height of the frustum = h = 15 cm


Capacity of the frustum






= 28490 cm3


= 28.49 litres



Question 7.

An open metal bucket is in the shape of a frustum of a cone, mounted on a hollow cylindrical base made of the same metallic sheet (see Fig.) The diameters of the two circular ends of the bucket are 45 cm and 25cm, the total vertical height of the bucket is 40cm and that of the cylindrical base is 6cm. Find the area of the metallic sheet used to make the bucket, where we do not take into account the handle of the bucket. Also, find the volume of water the bucket can hold. [Take π=22/7]



Answer:


Diameter of bigger circular end = 45cm


So,


Diameter of smaller circular end = 25cm


So,


Now,


Height of frustum = Total height of bucket – Height of cylinder


= 40 – 6


= 34cm


Also,


Slant height of the frustum, l = √{h2 + (R – r)2}




= √1156 + 100


= √ 1256


= 35.44cm


Curved Surface Area of frustum = πl(R + r)




= 22 × 35.44 × 5


= 3898.4 cm2


Curved Surface area of cylinder = 2πrh



= 471.428 cm2


Area of circular base = πr2



= 491.07 cm2


Now, Area of metallic sheet used


= CSA of frustum + Area of circular base + CSA of cylinder


= 3898.4 + 471.428 + 491.07


= 4860.89


= 4860.9 cm2






= 33615.47 cm3


We know that 1 cm3 = 0.001 litre


∴ Volume of water that bucket can hold = 33.62 litres (approx.)



Question 8.

A bucket made up of a metal sheet is in the form of frustum of a cone. Its depth is 24 cm and the diameters of the top and bottom are 30 cm and 10 cm respectively. Find the cost of milk which can completely fill the bucket at the rate of Rs. 20 per litre and the cost of metal sheet used if it costs Rs. 10 per 100cm2. [Use π=3.14]


Answer:


Greater diameter of the bucket = 30 cm


Radius of the bigger end of the bucket = R = 15 cm


Diameter of the smaller end of the bucket = 10 cm


Radius of the smaller end of the bucket = r = 5 cm


Height of the bucket = 24 cm


Slant height, l = √{h2 + (R – r)2}


= √{(24)2 +(15 – 5)2


= √576 + (10)2


= √576 + 100


= √676


= 26 cm


Now,




= 3.14 × 8 [225 + 25 + 75]


= 3.14 × 8 ×325


= 8164 cm3


= 8.164 Litres


A litre of milk cost Rs 20


So, total cost of filling the bucket with milk = Rs 20 × 8.164


= Rs 163.28


Surface Area of the bucket


= CSA of the frustum + Area of circular base


= πl(R+r) + πr2


= π [{26×(15+5)} + (5)2]


= π[520 + 25 ]


= 3.14 × 545


= 1711.3 cm2


Cost of 100cm2 of metal sheet = Rs 10




= Rs 171.13



Question 9.

A tent is made in the form of a conic frustum, surmounted by a cone. The diameters of the base and top of the frustum are 14m and 7 m and its height is 8 m. The height of the tent is 12m. Find the quantity of canvas required. [Take π=22/7]


Answer:


For the lower portion of the tent:


Diameter of the base = 14 m


Radius, R, of the base = 7 m


Diameter of the top end of the frustum = 7 m


Radius of the top end of the frustum =


Height of the frustum = h = 8 m


Slant height = l = √{h2 + (R – r)2}


= √{(8)2 +(7 – 3.5)2


= √64 + (3.5)2


= √64 + 12.25


= √76.25


= 8.73 m


For the conical part


Radius of the cone base = r = 3.5 m


Height of the cone = Height of the tent – height of frustum


= 12 – 8


= 4 m


Slant height of cone = L = √(4)2+(3.5)2


= √16 + 12.25


=√28.25


= 5.3 m


Total quantity of canvas = CSA of frustum + CSA of conical top


= πl(R + r) + πrL


= π [8.73(7 + 3.5) + 3.5 × 5.3]




= 346.5 m2



Question 10.

An oil funnel of tin sheet consists of a cylindrical portion 8 cm along attached to a frustum of a cone. If the total height be 15cm, the diameter of the cylindrical portion 1 cm and diameter of the top of the funnel 10cm, find the area of the tin required. [Take π=22/7]


Answer:


Diameter of top of funnel = 10cm


So, Radius (R) = 5cm


Diameter of cylindrical portion = 1cm


So,


Height of frustum = Total height – Height of cylindrical part


= 15 – 8


= 7cm


and Slant Height, l = √{h2 + (R – r)2}


= √{(7)2 +(5 – 0.5)2


= √49 + (4.5)2


= √49 + 20.25


= √69.25


= 8.32 cm


Now, CSA of frustum = πl (R + r)




= 143.81 cm2


Height of cylinder, H = 8cm


Now, CSA of cylinder = 2πrH



= 25.14 cm2


Area of tin required = CSA of frustum + CSA of cylinder


= 143.81 + 25.14


= 168.95 cm2