Freefall (physical)

See also: Freefall

A freefall is a movement accelerated under the only effect of the Pesanteur. One distinguishes the simple fall in a uniform field of gravity in the vicinity of the Ground (Galileo, 1605), and the celestial fall (Lois of Kepler), whose Sir Isaac Newton will make the synthesis in 1687.

It is agreed that the others forces acting on the body, are neglected (for more precise details, to see Déviation towards the East), in particular the resistance of the air. For the case where the resistance of the air is considered, one speaks about Chute with resistance of the air.

Close examples of freefalls

  • the freefall of Galileo (1602), since the Turn of Pisa.

  • the apple of Newton (1665) which falls from the tree, a very famous fable.
  • the experiment of the Tube of Newton.
  • an elevator which one would have cut the cable of suspension (Tour of weightlessness).
  • a caravel zero G .

By opposition , of the cases where other forces that the Pesanteur are also present and must be taken into account to describe the movement of the object, are presented hereafter:

  • object resting on a Surface, for example horizontal: case for which the force exerted by this surface compensates for the weight and acts so that the total force resulting force is null, not communicating acceleration with the object.

  • Plane or breaks into leaf of planing Papier: in these systems the forces of friction exerted by the air on the object play a fundamental role.

Fall slowed down

One also can, like made Galileo astutely, to operate a slowed down fall, for better observing the movement: fall of a metal disc on tilted glaze of an angle α, falls on successive levels, falls circular of the simple Pendule. Fall of the cycloidal Pendule of Huygens. Fall slowed down Machine of Atwood; it is obvious that the law of fall is different, but not the law of 1602: the mass m does not intervene, because there is exact compensation between inert mass and serious mass!

Freefall without initial speed

By supposing that the body is subjected only to gravity, if a specific body P is released of a point of dimension z0 without initial speed, then:

a_z = - g (component according to the axis of Z of acceleration, second law of Newton)

v_z = - WP + V_0 = - gt (component according to the axis of Z speed)
z = - \ frac {1} {2} gt^2+z_0 (component according to the axis of Z of the position)

With:

  • Z the height of the body compared to the ground
  • G the acceleration of the terrestrial field of gravity (approximately 9,81 m.s-2)
  • T time in second S

Speed V with the impact is given by:

V = \ sqrt {2gz_0}

Fall with initial speed

It will describe a parabolic Trajectoire (see also Parabole of safety).

Celestial fall

If initial speed is suitable, i.e. with the good value and the good orientation being given the initial position, the trajectory can be circular (cf Crenel), as it is it for a satellite geostationary; the the Moon rather has an elliptic movement (at first approximation), very disturbed by the influence of the Sun (the force of gravitation of the Sun on the Moon is larger than the Force of gravitation of the Earth on the Moon!).

Comment

In systems in simple freefall, the objects do not have a apparent Poids and float freely the ones compared to the others. For this reason, the concept of freefall is employed in the turns of weightlessness, the Avion S in compensated parabolic Vol (Caravel 0-g) or the systems in Orbite to simulate the absence of Gravité and to study its consequences.

Contrary to an generally accepted idea, the Spationaut S in a Space station, do not float in Apesanteur because of a reduction in the Gravitation due to their distance of the Ground, but because the system consisted the space station and themselves are in freefall (cf noninertial Référentiel, weightlessness).

The concept of freefall is approached in physics: the parabolic trajectory there is explained; then the nonparabolic ballistic trajectory, finally the trajectory of a satellite (laws of Kepler). Of course one points out that the reasoning of Torricelli (1640?) for the parabola always holds for the ellipse, the circular case being simplest to explain (Huygens, 1651): it is celebrates it figure of the " funicular with rochets" : tangent movement + fallen down " verticale" on the trajectory, etc Newton will use much this figure (1679-1687).

Relativistic vision

In the theory of the General relativity, the gravitation is not a force but a “deformation riemanienne of the space time”; an object called in freefall describes simply geodetic this space. It will be noticed that the law of Galileo (1602) the bodies have even law of fall was high with the row of Principe of equivalence (of the inert mass and the mass engraves) by Einstein in 1915, when it created his theory.

See too

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