If the radii of the circular ends of a bucket 28 cm high, are 28 cm and 7 cm, find its capacity and total surface area.
Explanation: Here the bucket is in the shape of a frustum. So capacity of bucket will be equal to the volume of the frustum and in order to calculate the total surface area of the bucket we will subtract the top end circular area from the total surface area of the frustum as the bucket is open on top.
Upper end radius of frustum/bucket = R = 28 cm
Lower end radius of frustum/bucket =r = 7 cm
Height of the frustum/bucket = 28 cm
And we know,
The capacity of bucket = Volume of the bucket
And, Volume of bucket = Volume of Frustum
∴ Capacity of bucket = Volume of frustum
⇒Volume of frustum
Where R = Radius of larger or upper end and r = Radius of smaller or lower end and h = height of frustum π = 22/7
⇒Volume of frustum
= 22 × 4 × 343
∴ Volume of frustum = 30184 cm3
∴ Capacity of bucket = 30184 cm3
T.S.A of bucket = T.S.A of frustum – Area of upper circle →eqn1
Let the slant height of the frustum be ‘ℓ’ cm
So, ℓ2 = h2 + (R – r)2
⇒ ℓ2 = 282 + (28 – 7)2
⇒ ℓ2 = 784 + (21)2
⇒ ℓ2 = 784 + 441
⇒ ℓ2 = 1225
⇒ ℓ = √(1225)
∴ ℓ = 35 cm
⇒ T.S.A of frustum = π(R + r)ℓ + πR2 + πr2
= π(28 + 7) × 35 + π(28)2 + π(7)2
= 35 × 35π + 784π + 49π
= 1225π + 784π + 49π →eqn2
Area of upper circle = πR2
= π(28)2
= 784π →eqn3
T.S.A of bucket = 1225π + 784π + 49π – 784π (from eqn2 and 3)
⇒ T.S.A of bucket = 1274π
⇒T.S.A of bucket=
⇒ T.S.A of bucket = 182 × 22
∴ T.S.A of bucket = 4004 cm2
The capacity and total surface area of the bucket is 30184 cm3 and 4004 cm2.