Overhead lines are common in both urban and rural areas. Someone put forward such a problem: When erecting the wire, the wire between the two poles should droop slightly. What is the principle of this?

First correct it: The drooping technical term is sag.

First of all, this sag is specifically designed by calculation, instead of being vertical by gravity, it is impossible to straighten it. It is only for the sake of straightness that it can be achieved only for other reasons. . What is the reason? Come and listen to the science student's interpretation.

From a physical point of view, the wires are made of metal materials, and the winter metal shrinks when it is cooled, and the stretched wires are broken by the contraction force. Metals and woods are all affected by thermal expansion and contraction. Although metals have prolonged properties, they can be elongated. However, at low temperatures, the elongation will decrease. Therefore, the wire will droop slightly to prevent the cold temperature from breaking.

Analyze from the angle of mechanics: For the cable analysis between two electric towers, the cable can only be pulled and the pulling direction is along the tangent. If the cable is completely horizontal, then the pulling force at both ends of the cable is horizontal to the left and horizontal to the right, resulting in vertical The direction of gravity is not balanced. The tighter the wire is, the smaller the angle between the tangent line and the pole that the pole is in contact with, and the greater the pull force along the wire direction to equalize the gravity of the wire = mg/sinθ, the more easily the wire will break. If the ideal situation can be straightened, θ tends to zero and the pull force approaches positive infinity.