The energy cost of transport quantifies the
energy efficiency
Energy efficiency may refer to:
* Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process
** Electrical efficiency, useful power output per electrical power consumed
** Mechanical efficiency, a ...
of transporting an animal or vehicle from one place to another. As a
dimensionless quantity
A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no physical dimension is assigned, with a corresponding SI unit of measurement of one (or 1) ...
, it allows for the comparison of dissimilar animals or
modes of transportation. It has a wide range of applications, from comparing human
gait
Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on speed, terrain, the need to maneuver, and energetic efficiency. ...
s to observing the change in efficiency of trains over time.
It is calculated in one of two ways, both shown in the following definition:
where
is the energy input to the system, which has mass
, that is used to move the system a distance
, and
is
Standard gravity
The standard acceleration due to gravity (or standard acceleration of free fall), sometimes abbreviated as standard gravity, usually denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. ...
. Alternatively, one can use the power input to the system
used to move the system at a constant velocity
. The cost of transport is non-dimensional.
It is also called specific tractive force or specific resistance (see
von Kármán–Gabrielli diagram), or the energy index. When the energy comes from
metabolic
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cel ...
processes (i.e., for animals), it is often called the metabolic cost of transport.
The metabolic cost of transport includes the
basal metabolic cost of maintaining bodily function, and so goes to infinity as speed goes to zero.
[
]
A human achieves the lowest cost of transport when walking at about , at which speed a person of has a metabolic rate of about 450
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Watt ...
s.
[ This gives a dimensionless cost of transport of about 0.39. If only the additional metabolic cost (above the resting rate) is counted then the most efficient speed will be lower. The optimal speed if both energy and distance traveled in a given time are taken into account (with some "price" for each) may be faster or slower than the speed giving the lowest COT.
Previously it was thought that a running person (unlike running animals) uses the same energy whether they run a distance slowly or fast. More recent experiments have shown that was mistaken. Cost of transport when running does depend on speed — individual people have an optimal running speed. ]
References
{{reflist
Transport economics