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A fall line refers to the line down a 
In
A fall line refers to the line down a mountain
A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher t ...
or hill
A hill is a landform that extends above the surrounding terrain. It often has a distinct summit, and is usually applied to peaks which are above elevation compared to the relative landmass, though not as prominent as Mountain, mountains. Hills ...
which is most directly downhill; that is, the direction a ball or other body would accelerate if it were free to move on the slope under gravity
In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...
. Mathematically the fall line, the line of greatest slope, is the negative of the gradient
In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...
(which points uphill) and perpendicular to the contour line
A contour line (also isoline, isopleth, isoquant or isarithm) of a Function of several real variables, function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a ...
s.
In mountain biking
Mountain biking (MTB) is a sport of riding bicycles off-road, often over rough terrain, usually using specially designed mountain bikes. Mountain bikes share similarities with other bikes but incorporate features designed to enhance durability ...
, a trail
A trail, also known as a path or track, is an unpaved lane or a small paved road (though it can also be a route along a navigable waterways) generally not intended for usage by motorized vehicles, usually passing through a natural area. Ho ...
follows the "fall line" if it generally descends in the most downward direction, rather than traversing in a sideways direction. A skier is said to be "skiing the fall line" if they are moving generally down, making turns either side of the fall line, rather than moving across the slope.
See also
*Glossary of cycling
This is a glossary of terms and jargon used in cycling, mountain biking, and cycle sport.
For ''parts of a bicycle'', see List of bicycle parts.
0–9
; 27.5 Mountain bike: A mountain bike with wheels that are approximately in diameter an ...
* Ridge line
A ridge is a long, narrow, elevated geomorphology, geomorphologic landform, Structural geology, structural feature, or a combination of both separated from the surrounding terrain by steep sides. The sides of a ridge slope away from a narrow ...
* Topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sci ...
* Topographic profile
A topographic profile or topographic cut or elevation profile is a representation of the relief of the terrain that is obtained by cutting transversely the lines of a topographic map. Each contour line can be defined as a closed line joining relie ...
* Gradient descent
Gradient descent is a method for unconstrained mathematical optimization. It is a first-order iterative algorithm for minimizing a differentiable multivariate function.
The idea is to take repeated steps in the opposite direction of the gradi ...
References
Mountain biking Alpine skiing Trails Topography {{Skiing-stub