A chemist has an empty cylinder with a height of 30 cm and a cone-shaped flask. The flask has the same height and a base that is the same size as the cylinder's. The flask is filled with water, which she pours into the cylinder. To what height does the water fill the cylinder?

20 cm
30 cm
15 cm
10 cm

1 answer

To determine the height to which the water fills the cylinder, we first need to understand the volumes of the cone-shaped flask and the cylinder.

Both the cylinder and the cone have the same base radius \( r \) and the same height \( h = 30 \text{ cm} \).

  1. Volume of the cylinder (V_cylinder): \[ V_{\text{cylinder}} = \pi r^2 h = \pi r^2 (30) \]

  2. Volume of the cone (V_cone): \[ V_{\text{cone}} = \frac{1}{3} \pi r^2 h = \frac{1}{3} \pi r^2 (30) = 10 \pi r^2 \]

Since the cone is filled with water, the volume of water in the cone is \( 10 \pi r^2 \).

Now, when the chemist pours this water into the cylinder, the volume of water in the cylinder will still be \( 10 \pi r^2 \).

Next, we need to determine how high \( h_{\text{water}} \) the water will fill the cylinder when it contains \( 10 \pi r^2 \) of water.

The volume of water in the cylinder when filled to height \( h_{\text{water}} \) is given by: \[ V_{\text{water}} = \pi r^2 h_{\text{water}} \]

Setting the two volumes equal gives us: \[ \pi r^2 h_{\text{water}} = 10 \pi r^2 \]

Dividing both sides by \( \pi r^2 \) (assuming \( r \neq 0 \)): \[ h_{\text{water}} = 10 \text{ cm} \]

Thus, the height to which the water fills the cylinder is 10 cm.