TOPICS

Exercise - 13.3

Surface Area and Volume

**Question-1 :-** A metallic sphere of radius 4.2 cm is melted and recast into the shape of a cylinder of radius 6 cm. Find the height of the cylinder. (Take π = 22/7)

Given that:- Radius(r_{1}) of the metallic sphere = 4.2 cm Radius(r_{2}) of the cylinder = 6 cm According to question, metalic sphere melted and recast into the shape of cylinder. So, Volume of sphere = Volume of cylinder 4/3 x πr_{1}^{3}= πr_{2}^{2}h 4/3 x r_{1}^{3}= r_{2}^{2}h 4/3 x 4.2 x 4.2 x 4.2 = 6 x 6 x h 98.784 = 36 x h h = 98.784/36 h = 2.744 cm Hence, height(h) of the cylinder = 2.744 cm

**Question-2 :-** Metallic spheres of radii 6 cm, 8 cm and 10 cm, respectively, are melted to form a single solid sphere. Find the radius of the resulting sphere. (Take π = 22/7)

Given that:- Radius(r_{1}) of metallic sphere = 6 cm Radius(r_{2}) of metallic sphere = 8 cm Radius(r_{3}) of metallic sphere = 10 cm Let R be the radius of resulting sphere. According to question, Three metallic sphere are melted to form a single solid sphere. So, Volume of resulting sphere = Volume of 1^{st}metallic sphere + Volume of 2^{nd}metallic sphere + Volume of 3^{rd}metallic sphere 4/3 x πR^{3}= 4/3 x πr_{1}^{3}+ 4/3 x πr_{2}^{3}+ 4/3 x πr_{3}^{3}R^{3}= r_{1}^{3}+ r_{2}^{3}+ r_{3}^{3}R^{3}= (6 x 6 x 6) + (8 x 8 x 8) + (10 x 10 x 10) R^{3}= 216 + 512 + 1000 R^{3}= 1728 R^{3}= (12)^{3}R = 12 cm Therefore, the radius of the sphere so formed will be 12 cm.

**Question-3 :-** A 20 m deep well with diameter 7 m is dug and the earth from digging is evenly spread out to form a platform 22 m by 14 m. Find the height of the platform. (Take π = 22/7)

Given that:- The shape of the well will be cylindrical. Diameter of the well = 7 m Radius(r) of the well = 7/2 m Depth(h) of the well = 20 m Area of platform = Length × Breadth = L x B = 22 × 14 m^{2}Let H be the height of the platform. Therefore, Volume of soil dug from the well will be equal to the volume of soil scattered on the platform. So, Volume of soil from well = Volume of soil used to make such platform πr^{2}h = L x B x H 22/7 x 7/2 x 7/2 x 20 = 22 x 14 x H H = 22/7 x 49/4 x 20 x 1/22 x 1/14 H = 5/2 m H = 2.5 m Therefore, the height of such platform will be 2.5 m.

**Question-4 :-** A well of diameter 3 m is dug 14 m deep. The earth taken out of it has been spread evenly all around it in the shape of a circular ring of width 4 m to form an embankment. Find the height of the embankment. (Take π = 22/7)

Give that:- The shape of the well will be cylindrical. Diameter of the well = 3 m Radius(r) of the well = 3/2 m Depth(h) of the well = 14 m Width of embankment = 4 m From the figure, it can be observed that our embankment will be in a cylindrical shape having outer radius(R) = 4 + 3/2 = 11/2 m and inner radius(r) = 3/2 m Let H be the height of the embankment. Volume of soil dug from well = Volume of earth used to form embankment πr^{2}h = π(R^{2}- r^{2})H 3/2 x 3/2 x 14 = [(11/2 x 11/2) - (3/2 x 3/2)] x H 63/2 = [121/4 - 9/4] x H 63/2 = 112/4 x H 63/2 = 28 x H H = 63/56 H = 9/8 H = 1.125 m

**Question-5 :-** A container shaped like a right circular cylinder having diameter 12 cm and height 15 cm is full of ice cream. The ice cream is to be filled into cones of height 12 cm and diameter 6 cm, having a hemispherical shape on the top. Find the number of such cones which can be filled with ice cream. (Take π = 22/7)

Give that:- Diameter of circular cylinder = 12 cm Radius(r_{1}) of the circular cylinder = 12/2 = 6 cm Height(h_{1}) of the cicular cylinder = 15 cm Height(h_{2}) of the cone = 12 cm Diameter of the cone = 6 cm Radius(r_{2}) of the cone = 6/2 = 3 cm Let n ice-cream cones be filled with ice-cream of the container. Therefore, Volume of ice-cream in cylinder = n × (Volume of 1 ice-cream cone + Volume of hemispherical shape on the top) πr_{1}^{2}h_{1}= n x [(1/3 x πr_{2}^{2}h_{2}) + (2/3 x πr_{2}^{3})] π(r_{1}^{2}h_{1}) = nπ x [(1/3 x r_{2}^{2}h_{2}) + (2/3 x r_{2}^{3})] (r_{1}^{2}h_{1}) = n x [(1/3 x r_{2}^{2}h_{2}) + (2/3 x r_{2}^{3})] 6 x 6 x 15 = n x [(1/3 x 3 x 3 x 12) + (2/3 x 3 x 3 x 3)] 540 = n x [36 + 18] 540 = n x 54 n = 540/54 n = 10 Therefore, 10 ice-cream cones can be filled with the ice-cream in the container.

**Question-6 :-** How many silver coins, 1.75 cm in diameter and of thickness 2 mm, must be melted to form a cuboid of dimensions 5.5 cm × 10 cm × 3.5 cm? (Use π = 22/7)

Given that:- Coins are cylindrical in shape. Diameter of coins = 1.75 cm Radius(r) of coins = 1.75/2 = 175/200 = 7/8 cm Thickness(h) of coins = 2 mm = 0.2 cm Let n coins be melted to form the required cuboids. Cuboid dimensions = 5.5 cm x 10 cm x 3.5 cm Therefore, Volume of n coins = Volume of cuboids n x πr^{2}h = l x b x h n x 22/7 x 7/8 x 7/8 x 0.2 = 5.5 x 10 x 3.5 n x 0.48125 = 192.5 n = 192.5/0.48125 n = 400 Therefore, the number of coins melted to form such a cuboid is 400.

**Question-7 :-** A cylindrical bucket, 32 cm high and with radius of base 18 cm, is filled with sand. This bucket is emptied on the ground and a conical heap of sand is formed. If the height of the conical heap is 24 cm, find the radius and slant height of the heap. (Take π = 22/7)

Given that:- Height(h_{1}) of the cylinderical bucket = 32 cm Radius(r_{1}) of the cylinderical bucket = 18 cm Height(h_{2}) of the conical heap = 24 cm Let the radius of the circular end of conical heap be r_{2}. The volume of sand in the cylindrical bucket will be equal to the volume of sand in the conical heap. Therefore, Volume of sand in the cylindrical bucket = Volume of sand in conical heap πr_{1}^{2}h_{1}= 1/3 x πr_{2}^{2}h_{2}r_{1}^{2}h_{1}= 1/3 x r_{2}^{2}h_{2}18 x 18 x 32 = 1/3 x r_{2}^{2}x 24 18 x 18 x 32 = r_{2}^{2}x 8 18 x 18 x 4 = r_{2}^{2}r_{2}^{2}= 1296 r_{2}^{2}= (36)^{2}r_{2}= 36 cm Slant height(l) of the conical heap = √r_{2}² + h_{2}² = √36^{2}+ 24^{2}= √1296 + 576 = √1872 = 12√13 cm Therefore, the radius and slant height of the conical heap are 36 cm and 12√13 cm respectively.

**Question-8 :-** Water in a canal, 6 m wide and 1.5 m deep, is flowing with a speed of 10 km/h. How much area will it irrigate in 30 minutes, if 8 cm of standing water is needed? (Take π = 22/7)

Given that:- Consider an area of cross-section of canal as ABCD. Area of cross-section = 6 × 1.5 = 9 m^{2}Speed of water = 10 km/h = 10000/60 = m/min Volume of water that flows in 1 minute from canal = 9 x 10000/60 = 15000 m^{3}Volume of water that flows in 30 minutes from canal = 30 × 1500 = 45000 m^{3}Let the irrigated area be A. Volume of water irrigating the required area will be equal to the volume of water that flowed in 30 minutes from the canal. Therefore, Vol. of water flowing in 30 minutes from canal = Vol. of water irrigating the required area 45000 = (A x 8)/100 A = 4500000/8 A = 562500 m^{2}Therefore, area irrigated in 30 minutes is 562500 m^{2}.

**Question-9 :-** A farmer connects a pipe of internal diameter 20 cm from a canal into a cylindrical tank in her field, which is 10 m in diameter and 2 m deep. If water flows through the pipe at the rate of 3 km/h, in how much time will the tank be filled? (Take π = 22/7)

Consider an area of cross-section of pipe as shown in the figure. Radius(r_{1}) of circular end of pipe = 20/200 = 0.1 m Area of cross-section = πr_{1}^{2}= π x 0.1 x 0.1 = 0.01π m^{2}Speed of water = 3 km/h = 3000/60 = 50 m/min Volume of water that flows in 1 minute from pipe = 50 × 0.01π = 0.5π m^{3}Volume of water that flows in t minutes from pipe = t × 0.5π m^{3}Radius(r_{2}) of circular end of cylindrical tank = 10/2 = 5 m Depth(h_{2}) of cylindrical tank = 2 m Let the tank be filled completely in t minutes. Volume of water filled in tank in t minutes is equal to the volume of water flowed in t minutes from the pipe. Therefore, Volume of water that flows in t minutes from pipe = Volume of water in tank Volume of water that flows in t minutes from pipe = Volume of water in tank t × 0.5π = π(r_{2})^{2}h_{2}t × 0.5 = 5^{2}× 2 t = 100 Therefore, the cylindrical tank will be filled in 100 minutes.

CLASSES