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The tidal force is an apparent force that stretches a body towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for the diverse phenomena, including tides, tidal locking, breaking apart of celestial bodies and formation of ring systems within Roche limit, and in extreme cases, spaghettification of objects. It arises because the gravitational force exerted on one body by another is not constant across its parts: the nearest side is attracted more strongly than the farthest side. It is this difference that causes a body to get stretched. Thus, the tidal force is also known as the differential force, as well as a secondary effect of the gravitational force. In celestial mechanics, the expression "tidal force" can refer to a situation in which a body or material (for example, tidal water) is mainly under the gravitational influence of a second body (for example, the Earth), but is also perturbed by the gravitational effects of a third body (for example, the Moon). The perturbing force is sometimes in such cases called a tidal force[1] (for example, the perturbing force on the Moon): it is the difference between the force exerted by the third body on the second and the force exerted by the third body on the first.[2]

Contents

1 Explanation 2 Effects of tidal forces 3 Mathematical treatment 4 See also 5 References 6 External links

Explanation[edit]

Figure 2: The Moon's gravity differential field at the surface of the Earth
Earth
is known (along with another and weaker differential effect due to the Sun) as the Tide
Tide
Generating Force. This is the primary mechanism driving tidal action, explaining two tidal equipotential bulges, and accounting for two high tides per day. In this figure, the Earth
Earth
is the central blue circle while the Moon
Moon
is far off to the right. The outward direction of the arrows on the right and left indicates that where the Moon
Moon
is overhead (or at the nadir) its perturbing force opposes that between the earth and ocean.

When a body (body 1) is acted on by the gravity of another body (body 2), the field can vary significantly on body 1 between the side of the body facing body 2 and the side facing away from body 2. Figure 2 shows the differential force of gravity on a spherical body (body 1) exerted by another body (body 2). These so-called tidal forces cause strains on both bodies and may distort them or even, in extreme cases, break one or the other apart.[3] The Roche limit
Roche limit
is the distance from a planet at which tidal effects would cause an object to disintegrate because the differential force of gravity from the planet overcomes the attraction of the parts of the object for one another.[4] These strains would not occur if the gravitational field were uniform, because a uniform field only causes the entire body to accelerate together in the same direction and at the same rate. Effects of tidal forces[edit]

Figure 3: Saturn's rings are inside the orbits of its principal moons. Tidal forces oppose gravitational coalescence of the material in the rings to form moons.[5]

In the case of an infinitesimally small elastic sphere, the effect of a tidal force is to distort the shape of the body without any change in volume. The sphere becomes an ellipsoid with two bulges, pointing towards and away from the other body. Larger objects distort into an ovoid, and are slightly compressed, which is what happens to the Earth's oceans under the action of the Moon. The Earth
Earth
and Moon
Moon
rotate about their common center of mass or barycenter, and their gravitational attraction provides the centripetal force necessary to maintain this motion. To an observer on the Earth, very close to this barycenter, the situation is one of the Earth
Earth
as body 1 acted upon by the gravity of the Moon
Moon
as body 2. All parts of the Earth
Earth
are subject to the Moon's gravitational forces, causing the water in the oceans to redistribute, forming bulges on the sides near the Moon
Moon
and far from the Moon.[6] When a body rotates while subject to tidal forces, internal friction results in the gradual dissipation of its rotational kinetic energy as heat. In the case for the Earth, and Earth's Moon, the loss of rotational kinetic energy results in a gain of about 2 milliseconds per century. If the body is close enough to its primary, this can result in a rotation which is tidally locked to the orbital motion, as in the case of the Earth's moon. Tidal heating
Tidal heating
produces dramatic volcanic effects on Jupiter's moon Io. Stresses caused by tidal forces also cause a regular monthly pattern of moonquakes on Earth's Moon.[7] Tidal forces contribute to ocean currents, which moderate global temperatures by transporting heat energy toward the poles. It has been suggested that in addition to other factors, harmonic beat variations in tidal forcing may contribute to climate changes. However, no strong link has been found to date.[8] Tidal effects become particularly pronounced near small bodies of high mass, such as neutron stars or black holes, where they are responsible for the "spaghettification" of infalling matter. Tidal forces create the oceanic tide of Earth's oceans, where the attracting bodies are the Moon
Moon
and, to a lesser extent, the Sun. Tidal forces are also responsible for tidal locking, tidal acceleration, and tidal heating. Tides may also induce seismicity. By generating conducting fluids within the interior of the Earth, tidal forces also affect the Earth's magnetic field.[9] Mathematical treatment[edit]

Tidal force
Tidal force
is responsible for the merge of galactic pair MRK 1034.[10]

Figure 4: Graphic of tidal forces. The top picture shows the gravity field of a body to the right, the lower shows their residual once the field at the centre of the sphere is subtracted; this is the tidal force. See Figure 2 for a more detailed version

For a given (externally generated) gravitational field, the tidal acceleration at a point with respect to a body is obtained by vectorially subtracting the gravitational acceleration at the center of the body (due to the given externally generated field) from the gravitational acceleration (due to the same field) at the given point. Correspondingly, the term tidal force is used to describe the forces due to tidal acceleration. Note that for these purposes the only gravitational field considered is the external one; the gravitational field of the body (as shown in the graphic) is not relevant. (In other words, the comparison is with the conditions at the given point as they would be if there were no externally generated field acting unequally at the given point and at the center of the reference body. The externally generated field is usually that produced by a perturbing third body, often the Sun
Sun
or the Moon
Moon
in the frequent example-cases of points on or above the Earth's surface in a geocentric reference frame.) Tidal acceleration
Tidal acceleration
does not require rotation or orbiting bodies; for example, the body may be freefalling in a straight line under the influence of a gravitational field while still being influenced by (changing) tidal acceleration. By Newton's law of universal gravitation
Newton's law of universal gravitation
and laws of motion, a body of mass m at distance R from the center of a sphere of mass M feels a force

F →

g

displaystyle vec F _ g

,

F →

g

= −

r ^

  G  

M m

R

2

displaystyle vec F _ g =- hat r ~G~ frac Mm R^ 2

equivalent to an acceleration

a →

g

displaystyle vec a _ g

,

a →

g

= −

r ^

  G  

M

R

2

displaystyle vec a _ g =- hat r ~G~ frac M R^ 2

where

r ^

displaystyle hat r

is a unit vector pointing from the body M to the body m (here, acceleration from m towards M has negative sign). Consider now the acceleration due to the sphere of mass M experienced by a particle in the vicinity of the body of mass m. With R as the distance from the center of M to the center of m, let ∆r be the (relatively small) distance of the particle from the center of the body of mass m. For simplicity, distances are first considered only in the direction pointing towards or away from the sphere of mass M. If the body of mass m is itself a sphere of radius ∆r, then the new particle considered may be located on its surface, at a distance (R ± ∆r) from the centre of the sphere of mass M, and ∆r may be taken as positive where the particle's distance from M is greater than R. Leaving aside whatever gravitational acceleration may be experienced by the particle towards m on account of m's own mass, we have the acceleration on the particle due to gravitational force towards M as:

a →

g

= −

r ^

  G  

M

( R ± Δ r

)

2

displaystyle vec a _ g =- hat r ~G~ frac M (Rpm Delta r)^ 2

Pulling out the R2 term from the denominator gives:

a →

g

= −

r ^

  G  

M

R

2

 

1

( 1 ± Δ r

/

R

)

2

displaystyle vec a _ g =- hat r ~G~ frac M R^ 2 ~ frac 1 (1pm Delta r/R)^ 2

The Maclaurin series
Maclaurin series
of

1

/

( 1 ± x

)

2

displaystyle 1/(1pm x)^ 2

is

1 ∓ 2 x + 3

x

2

∓ ⋯

displaystyle 1mp 2x+3x^ 2 mp cdots

which gives a series expansion of:

a →

g

= −

r ^

  G  

M

R

2

±

r ^

  G  

2 M

R

2

 

Δ r

R

+ ⋯

displaystyle vec a _ g =- hat r ~G~ frac M R^ 2 pm hat r ~G~ frac 2M R^ 2 ~ frac Delta r R +cdots

The first term is the gravitational acceleration due to M at the center of the reference body

m

displaystyle m

, i.e., at the point where

Δ r

displaystyle Delta r

is zero. This term does not affect the observed acceleration of particles on the surface of m because with respect to M, m (and everything on its surface) is in free fall. When the force on the far particle is subtracted from the force on the near particle, this first term cancels, as do all other even-order terms. The remaining (residual) terms represent the difference mentioned above and are tidal force (acceleration) terms. When ∆r is small compared to R, the terms after the first residual term are very small and can be neglected, giving the approximate tidal acceleration

a →

t

displaystyle vec a _ t

(axial) for the distances ∆r considered, along the axis joining the centers of m and M:

a →

t

displaystyle vec a _ t

(axial)

  ≈   ±  

r ^

  2 Δ r   G  

M

R

3

displaystyle ~approx ~pm ~ hat r ~2Delta r~G~ frac M R^ 3

When calculated in this way for the case where ∆r is a distance along the axis joining the centers of m and M,

a →

t

displaystyle vec a _ t

is directed outwards from to the center of m (where ∆r is zero). Tidal accelerations can also be calculated away from the axis connecting the bodies m and M, requiring a vector calculation. In the plane perpendicular to that axis, the tidal acceleration is directed inwards (towards the center where ∆r is zero), and its magnitude is

a →

t

displaystyle vec a _ t

(axial)

/

2

displaystyle /2

in linear approximation as in Figure 2. The tidal accelerations at the surfaces of planets in the Solar System are generally very small. For example, the lunar tidal acceleration at the Earth's surface along the Moon- Earth
Earth
axis is about 1.1 × 10−7 g, while the solar tidal acceleration at the Earth's surface along the Sun- Earth
Earth
axis is about 0.52 × 10−7 g, where g is the gravitational acceleration at the Earth's surface. Hence the tide-raising force (acceleration) due to the Sun
Sun
is about 45% of that due to the Moon.[11] The solar tidal acceleration at the Earth's surface was first given by Newton in the Principia.[12] See also[edit]

Tidal tensor Amphidromic point Disrupted planet Galactic tide Tidal resonance Spacetime curvature

References[edit]

^ "On the tidal force", I. N. Avsiuk, in "Soviet Astronomy Letters", vol. 3 (1977), pp. 96–99. ^ See p. 509 in "Astronomy: a physical perspective", M. L. Kutner (2003). ^ R Penrose (1999). The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics. Oxford University Press. p. 264. ISBN 0-19-286198-0.  ^ Thérèse Encrenaz; J -P Bibring; M Blanc (2003). The Solar System. Springer. p. 16. ISBN 3-540-00241-3.  ^ R. S. MacKay; J. D. Meiss (1987). Hamiltonian Dynamical Systems: A Reprint Selection. CRC Press. p. 36. ISBN 0-85274-205-3.  ^ Rollin A Harris (1920). The Encyclopedia Americana: A Library of Universal Knowledge. 26. Encyclopedia Americana Corp. pp. 611–617.  ^ "The Tidal Force Neil deGrasse Tyson". www.haydenplanetarium.org. Retrieved 2016-10-10.  ^ "Millennial Climate Variability: Is There a Tidal Connection?".  ^ "Hungry for Power in Space". New Scientist. New Science Pub. 123: 52. 23 September 1989. Retrieved 14 March 2016.  ^ "Inseparable galactic twins". ESA/Hubble Picture of the Week. Retrieved 12 July 2013.  ^ The Admiralty (1987). Admiralty manual of navigation. 1. The Stationery Office. p. 277. ISBN 0-11-772880-2. , Chapter 11, p. 277 ^ Newton, Isaac (1729). The mathematical principles of natural philosophy. 2. p. 307. ISBN 0-11-772880-2. , Book 3, Proposition 36, Page 307 Newton put the force to depress the sea at places 90 degrees distant from the Sun
Sun
at "1 to 38604600" (in terms of g), and wrote that the force to raise the sea along the Sun- Earth
Earth
axis is "twice as great", i.e. 2 to 38604600, which comes to about 0.52 × 10−7 g as expressed in the text.

External links[edit]

Gravitational Tides by J. Christopher Mihos of Case Western Reserve University Audio: Cain/Gay – Astronomy Cast Tidal Forces – July 2007. Gray, Meghan; Merrifield, Michael. "Tidal Forces". Sixty Symbols. Brady Haran
Brady Haran
for the University of Nottingham.  "Pau Amaro Seoane MODEST working group 4 "Tidal disruption of a star by a massive black hole"". Retrieved 2013-05-30.  Myths about Gravity and Tides by Mikolaj Sawicki of John A. Logan College and the University of Colorado. Tidal Misconceptions by Donald E. Simanek

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