Why are dams always made in the shape of an arc?

Answer

Concrete and stone structures can withstand a very large compressive force, but a limited tensile force. Due to the arc shape, the force of the water is completely absorbed as a pressure force. If the wall were straight, the force of the water could pull the wall open.

Try it yourself: line up five of your fellow students in a row, the arms hooked together: if you make a straight row and you push against the middle one, you have pushed the row apart in no time. If you set up the row in an arc shape, the middle one can lean against its colleagues and you won’t be able to push the row apart.