Question

Rani has several identical solid right circular metal cylinders of unknown base radius and height 10 cm. To find the base radius r of a cylinder, she puts them one by one into the above container half filled with water. When exactly 25 of them are put, the water reaches the level of the container being completely filled.


`show \: that \: r = 5 \sqrt{ (5)/(\pi) } cm`

Find the value of r in centimetres to the first decimal place, by using 3.14 for the value of

`\pi`
​

Answer 1

The volume of each cylinder is given by the formula: V = πr^2h, where r is the base radius and h is the height of the cylinder.

When 25 cylinders are put into the half-filled container, the water level rises to the top of the container, which means that the total volume of the 25 cylinders is equal to the volume of the container. Let's assume the volume of the container is V_container.

So, 25 * πr^2 * h = V_container

Dividing both sides by 25πh gives: r^2 = V_container / (25πh)

Taking the square root of both sides gives: r = √(V_container / (25πh))

Since h = 10 cm, we can substitute this value in the formula above: r = √(V_container / (25π * 10))

Since r is the base radius of the cylinder, it must be positive. So, the final equation becomes:

r = √(V_container / (25π * 10)) cm = 5√(5/π) cm. shown

By using 3.14 for the value of π, we can calculate the value of r:

r = 5√(5/π) = 5√(5/3.14) = 5 * √(5/3.14)

= 5 * √(1.5873) = 5 * 1.259 = 6.295 cm (rounded to the first decimal place)

So, the base radius of the cylinder is approximately 6.3 cm.