thumb|[[Wheelchair ramp, Hotel Montescot, Chartres, France]]

thumb|Demonstration inclined plane used in education, [[Museo Galileo, Florence.]]

An inclined plane, also known as a ramp, is a flat supporting surface tilted at an angle from the vertical direction, with one end higher than the other, used as an aid for raising or lowering a load. The inclined plane is one of the six classical simple machines defined by Renaissance scientists. Inclined planes are used to move heavy loads over vertical obstacles. Examples vary from a ramp used to load goods into a truck, to a person walking up a pedestrian ramp, to an automobile or railroad train climbing a grade. The mechanical advantage of an inclined plane, the factor by which the force is reduced, is equal to the ratio of the length of the sloped surface to the height it spans. Owing to conservation of energy, the same amount of mechanical energy (work) is required to lift a given object by a given vertical distance, disregarding losses from friction, but the inclined plane allows the same work to be done with a smaller force exerted over a greater distance.

The angle of friction, also sometimes called the angle of repose, is the maximum angle at which a load can rest motionless on an inclined plane due to friction without sliding down. This angle is equal to the arctangent of the coefficient of static friction μ<sub>s</sub> between the surfaces. The wedge can be considered a moving inclined plane or two inclined planes connected at the base. He imagined two inclined planes of equal height but different slopes, placed back-to-back as in a prism (A, B, C above). A loop of string with beads at equal intervals is draped over the inclined planes, with part of the string hanging down below. The beads resting on the planes act as loads on the planes, held up by the tension force in the string at point T. Stevin's argument goes like this:

  • The string must be stationary, in static equilibrium. If the string was heavier on one side than the other, and began to slide right or left under its own weight, when each bead had moved to the position of the previous bead the string would be indistinguishable from its initial position and therefore would continue to be unbalanced and slide. This argument could be repeated indefinitely, resulting in a circular perpetual motion, which is absurd. Therefore, it is stationary, with the forces on the two sides at point T (above) equal.
  • The portion of the chain hanging below the inclined planes is symmetrical, with an equal number of beads on each side. It exerts an equal force on each side of the string. Therefore, this portion of the string can be cut off at the edges of the planes (points S and V), leaving only the beads resting on the inclined planes, and this remaining portion will still be in static equilibrium.
  • Since the beads are at equal intervals on the string, the total number of beads supported by each plane, the total load, is proportional to the length of the plane. Since the input supporting force, the tension in the string, is the same for both, the mechanical advantage of each plane is proportional to its slant length...

As pointed out by Dijksterhuis, Stevin's argument is not completely tight. The forces exerted by the hanging part of the chain need not be symmetrical because the hanging part need not retain its shape when let go. Even if the chain is released with a zero angular momentum, motion including oscillations is possible unless the chain is initially in its equilibrium configuration, a supposition which would make the argument circular.

|}

Inclined planes have been used by people since prehistoric times to move heavy objects. The sloping roads and causeways built by ancient civilizations such as the Romans are examples of early inclined planes that have survived, and show that they understood the value of this device for moving things uphill. The heavy stones used in ancient stone structures such as Stonehenge are believed to have been moved and set in place using inclined planes made of earth, although it is hard to find evidence of such temporary building ramps. The Egyptian pyramids were constructed using inclined planes, Siege ramps enabled ancient armies to surmount fortress walls. The ancient Greeks constructed a paved ramp 6&nbsp;km (3.7 miles) long, the Diolkos, to drag ships overland across the Isthmus of Corinth. This view persisted among a few later scientists; as late as 1826 Karl von Langsdorf wrote that an inclined plane "...is no more a machine than is the slope of a mountain". The problem of calculating the force required to push a weight up an inclined plane (its mechanical advantage) was attempted by Greek philosophers Heron of Alexandria (c. 10 - 60 CE) and Pappus of Alexandria (c. 290 - 350 CE), but their solutions were incorrect.

It was not until the Renaissance that the inclined plane was solved mathematically and classed with the other simple machines. The first correct analysis of the inclined plane appeared in the work of 13th century author Jordanus de Nemore, however his solution was apparently not communicated to other philosophers of the time.

The first elementary rules of sliding friction on an inclined plane were discovered by Leonardo da Vinci (1452-1519), but remained unpublished in his notebooks. They were rediscovered by Guillaume Amontons (1699) and were further developed by Charles-Augustin de Coulomb (1785).

Terminology

Slope

The mechanical advantage of an inclined plane depends on its slope, meaning its gradient or steepness. The smaller the slope, the larger the mechanical advantage, and the smaller the force needed to raise a given weight. A plane's slope s is equal to the difference in height between its two ends, or "rise", divided by its horizontal length, or "run". It can also be expressed by the angle the plane makes with the horizontal, <math>\theta</math>.

thumb|The inclined plane's geometry is based on a [[right triangle.

Frictionless inclined plane

right|thumb|Instrumented inclined plane used for physics education, around 1900. The lefthand weight provides the load force <math>F_\text{w}</math>. The righthand weight provides the input force <math>F_\text{i}</math> pulling the roller up the plane.

If there is no friction between the object being moved and the plane, the device is called an ideal inclined plane. This condition might be approached if the object is rolling like a barrel, or supported on wheels or casters. Due to conservation of energy, for a frictionless inclined plane the work done on the load lifting it, <math>W_\text{out}</math>, is equal to the work done by the input force, <math>W_\text{in}</math>

:<math>W_{\rm out} = W_{\rm in} \,</math>

Work is defined as the force multiplied by the displacement an object moves. The work done on the load is equal to its weight multiplied by the vertical displacement it rises, which is the "rise" of the inclined plane

:<math>W_{\rm out} = F_{\rm w} \cdot \text{Rise} \,</math>

The input work is equal to the force <math>F_\text{i}</math> on the object times the diagonal length of the inclined plane.

:<math>W_{\rm in} = F_{\rm i} \cdot \text{Length} \,</math>

Substituting these values into the conservation of energy equation above and rearranging

:<math>\text{MA} = \frac{F_{\rm w{F_{\rm i = \frac {\text{Length{\text{Rise \,</math>

To express the mechanical advantage by the angle <math>\theta</math> of the plane, The maximum friction force is given by

:<math>F_f = \mu F_n \,</math>

where <math>F_\text{n}</math> is the normal force between the load and the plane, directed normal to the surface, and <math>\mu</math> is the coefficient of static friction between the two surfaces, which varies with the material. When no input force is applied, if the inclination angle <math>\theta</math> of the plane is less than some maximum value <math>\phi</math> the component of gravitational force parallel to the plane will be too small to overcome friction, and the load will remain motionless. This angle is called the angle of repose and depends on the composition of the surfaces, but is independent of the load weight. It is shown below that the tangent of the angle of repose <math>\phi</math> is equal to <math>\mu</math>

:<math>\phi = \tan^{-1} \mu \,</math>

With friction, there is always some range of input force <math>F_\text{i}</math> for which the load is stationary, neither sliding up or down the plane, whereas with a frictionless inclined plane there is only one particular value of input force for which the load is stationary.

Analysis

right|thumb|Key: F<sub>n</sub> = N = [[Normal force that is perpendicular to the plane, F<sub>i</sub> = f = input force, F<sub>w</sub> = mg = weight of the load, where m = mass, g = gravity ]]

A load resting on an inclined plane, when considered as a free body has three forces acting on it: