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Weightlessness - Wikipedia. A block of lead in free fall on planet X.

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The block is said to be in a state of weightlessness although being pulled down by the planet's gravity. The two bodies are GR inertial and are accelerating towards each other. Counterintuitively, a uniform gravitational field does not by itself cause stress or strain, and a body in free fall in such an environment experiences no g- force acceleration and feels weightless. In such cases, a sensation of weight, in the sense of a state of stress can occur, even if the gravitational field were zero. In such cases, g- forces are felt, and bodies are not weightless. When the gravitational field is non- uniform, a body in free fall suffers tidal effects and is not stress- free.

Near a black hole, such tidal effects can be very strong. In the case of the Earth, the effects are minor, especially on objects of relatively small dimension (such as the human body or a spacecraft) and the overall sensation of weightlessness in these cases is preserved. This condition is known as microgravity and it prevails in orbiting spacecraft.

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In October 2. 01. NASA Office of Inspector General issued a health hazards report related to human spaceflight, including a human mission to Mars. In the right half, it is in a uniform gravitation field. Near the surface of the earth, a body whose mass is 1 kg has a weight of approximately 9. N, independent of its state of motion, free fall, or not. Weightlessness in this sense can be achieved by removing the body far away from the source of gravity. It can also be attained by placing the body at a neutral point between two gravitating masses.

Weight. 2: Weight can also be interpreted as that quantity which is measured when one uses scales. What is being measured there is the force exerted by the body on the scales. In a standard weighing operation, the body being weighed is in a state of equilibrium as a result of a force exerted on it by the weighing machine cancelling the gravitational field. By Newton's 3rd law, there is an equal and opposite force exerted by the body on the machine. This force is called weight. The force is not gravitational. Typically, it is a contact force and not uniform across the mass of the body.

If the body is placed on the scales in a lift (an elevator) in free fall in pure uniform gravity, the scale would read zero, and the body said to be weightless i. This describes the condition in which the body is stress free and undeformed. This is the weightlessness in free fall in a uniform gravitational field. See below.)To sum up, we have two notions of weight of which weight. Yet 'weightlessness' is typically exemplified not by absence of weight. This is the intended sense of weightlessness in what follows below.

A body is stress free, exerts zero weight. Without subscripts, one ends up with the odd- sounding conclusion that a body is weightless when the only force acting on it is its weight. The apocryphal apple that fell on Newton's head can be used to illustrate the issues involved.

An apple weighs approximately 1 newton. This is the weight.

During that fall, its weight. When it hits Newton, the sensation felt by Newton would depend upon the height from which the apple falls and weight. N. It was great enough—in the story—to make the great man invent the theory of gravity. It is this weight.

On its way down, the apple in its free fall does not suffer any distortion as the gravitational field is uniform. Stress during free fall. In a uniform gravitational field: Consider any cross- section dividing the body into two parts. Both parts have the same acceleration and the force exerted on each is supplied by the external source of the field. There is no force exerted by one part on the other. Stress at the cross- section is zero. In a non- uniform gravitational field: Under gravity alone, one part of the body may have a different acceleration from another part.

This would tend to deform the body and generate internal stresses if the body resists deformation. Weight. 2 is not 0.

Throughout this discussion on using stress as an indicator of weight, any pre- stress which may exist within a body caused by a force exerted on one part by another is not relevant. The only relevant stresses are those generated by external forces applied to the body. The definition and use of 'weightlessness' is difficult unless it is understood that the sensation of . An object in a straight free fall, or in a more complex inertial trajectory of free fall (such as within a reduced gravity aircraft or inside a space station), all experience weightlessness, since they do not experience the mechanical forces that cause the sensation of weight. Force fields other than gravity.

This is that a body may be subject to a field which is not gravitational but such that the force on the object is uniformly distributed across the object's mass. An electrically charged body, uniformly charged, in a uniform electric field is a possible example. Electric charge here replaces the usual gravitational charge. Such a body would then be stress free and be classed as weightless. Various types of levitation may fall into this category, at least approximately. Weightlessness and proper acceleration. If the body is in a freely falling lift and subject to no pushes or pulls from the lift or its contents, the acceleration with respect to the lift would be zero.

If on the other hand, the body is subject to forces exerted by other bodies within the lift, it will have an acceleration with respect to the freely falling lift. This acceleration which is not due to gravity is called . On this approach, weightlessness holds when proper acceleration is zero.

How to avoid weightlessness. Aero- dynamic lift, drag, and thrust are all non- uniform forces (they are applied at a point or surface, rather than acting on the entire mass of an object), and thus create the phenomenon of weight. This non- uniform force may also be transmitted to an object at the point of contact with a second object, such as the contact between the surface of the Earth and one's feet, or between a parachute harness and one's body. Tidal forces. The string stretches as the body falls to the right. Tidal forces arise when the gravitational field is not uniform and gravitation gradients exist. Such indeed is the norm and strictly speaking any object of finite size even in free- fall is subject to tidal effects.

These are impossible to remove by inertial motion, except at one single nominated point of the body. The Earth is in free fall but the presence of tides indicates that it is in a non- uniform gravitational field. This non- uniformity is more due to the moon than the sun. The total gravitational field due to the sun is much stronger than that of the moon but it has a minor tidal effect compared with that of the moon because of the relative distances involved. Weight. 1 of the earth is essentially due to the sun's gravity.

But its state of stress and deformation, represented by the tides, is more due to non uniformity in the gravitational field of the nearby moon. When the size of a region being considered is small relative to its distance from the gravitating mass the assumption of uniform gravitational field holds to a good approximation. Thus a person is small relative to the radius of Earth and the field for a person at the surface of the earth is approximately uniform. The field is strictly not uniform and is responsible for the phenomenon of microgravity. Objects near a black hole are subject to a highly non- uniform gravitational field.

Frames of reference. Inside the frame (for example, inside an orbiting ship or free- falling elevator), unforced objects keep their velocity relative to the frame. Objects not in contact with other objects .

If the inertial trajectory is influenced by gravity, the reference frame will be an accelerated frame as seen from a position outside the gravitational attraction, and (seen from far away) the objects in the frame (elevator, etc.) will appear to be under the influence of a force (the so- called force of gravity). As noted, objects subject solely to gravity do not feel its effects. Weightlessness can thus be realised for short periods of time in an airplane following a specific elliptic flight path, often mistakenly called a parabolic flight. It is simulated poorly, with many differences, in neutral buoyancy conditions, such as immersion in a tank of water. Zero- g, . Zero- g is subtly different from the complete absence of gravity, something which is impossible due to the presence of gravity everywhere in the universe. Accelerometers can only detect g- force i. They cannot detect the acceleration associated with free fall.

In the absence of this force, a person would be in free- fall, and would experience weightlessness. It is the transmission of this reaction force through the human body, and the resultant compression and tension of the body's tissues, that results in the sensation of weight. Because of the distribution of mass throughout a person's body, the magnitude of the reaction force varies between a person's feet and head. At any horizontal cross- section of a person's body (as with any column), the size of the compressive force being resisted by the tissues below the cross- section is equal to the weight of the portion of the body above the cross- section. In the pose adopted in the accompanying illustration, the shoulders carry the weight of the outstretched arms and are subject to a considerable torque.

Watch El Prestamista online in english with english subtitles in 1080p here. A common misconception. Although there is a way of making sense of this within the physics of Einstein's general relativity, within Newtonian physics, this is technically inaccurate . An observer on the marked spot will see the satellite remain directly overhead unlike the other heavenly objects which sweep across the sky. Spacecraft are held in orbit by the gravity of the planet which they are orbiting.