A dam is able to hold a large quantity of water because its wall is

Analyze the question: The question asks why a dam wall has a particular shape. This is about liquid pressure.

Concept: Pressure in a liquid increases with depth:

P = ρgh

where ρ = density of the liquid, g = acceleration due to gravity, and h = depth below the surface.

So the deeper you go, the greater the pressure the water exerts on the wall.

Solution:

1. At the top of the dam, the depth h is small, so the pressure is small.
2. At the bottom, the depth h is greatest, so the pressure is greatest.
3. The wall must withstand the greatest force where the pressure is greatest, so it must be strongest, meaning thickest, at the bottom.
4. If the wall were thickest at the top or the middle, or the same thickness throughout, the base could give way under the high pressure and the dam could burst.

Checking the options:

• A. Thickest at the top: wrong, since the pressure is lowest there.
• B. Thickest at the middle: wrong, since the maximum pressure is at the base.
• C. Thickest at the bottom: correct.
• D. Same thickness throughout: wrong, since it ignores how pressure varies with depth.

Exam tip: Remember "deeper means higher pressure, so thicker base." The same idea explains why submarines must be strongly built for deep dives and why water from a hole low in a container shoots out farther than from a hole near the top.