An Etymological Dictionary of Astronomy and Astrophysics
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1) The cause of a change in motion or shape of a body.
2) A → vector quantity defined by → Newton's second law as the rate of change of → momentum or, equivalently, as the product of → mass and → acceleration. A force is completely specified by giving its magnitude, direction, and point of application. In mechanics, force represents the quantitative measure of the mechanical interaction of material bodies.

From O.Fr. force, from L.L. fortia, from neut. pl. of L. fortis "strong," from forctus, from PIE base *bhergh- "high" (cf. Av. barəz- "high, mount," barezan- "height;" O.Pers. baršan- "height;" Mod.Pers. borz in (the mountain chain) Alborz, and borz "height, magnitude," bâlâ "up, above, high, elevated, height," Lori dialect berg "hill, mountain;" Skt. bhrant- "high;" O.E. burg, burh "castle, fortified place," from P.Gmc. *burgs "fortress;" Ger. Burg "castle," Goth. baurgs "city," E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg).

Niru, from Mid.Pers. nêrok, Av. nairya- "manly, male" (cf. Skt. nárya-), from nar- "man, male" (Mid./Mod.Pers. nar- "male," Skt. nár- "male").

Same as → field of force.

→ force; → field.

One of the three quantities (k, α, and δ) which are used in the → radiation-driven wind theory to express the radiation pressure due to spectral lines. These coefficients parametrize the radiation acceleration as: g_rad≅ k(dv/dr)^αg_e, where g_e = σ_eL/4πcR_*² is the radiative acceleration by electron scattering. The parameter k is dependent on the number of lines that produce the radiation pressure. The parameter α depends on the optical depth of the driving lines and varies between 0 (optically thin) and 1 (optically thick). The parameter δ describes the dependence of k on the density with k ≅ ρ^δ. The velocity law of radiation-driven winds depends on α and δ, but not on k. The → mass loss rate depends on k, α, and δ (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269 and references therein).

→ force; → multiplier; → parameter.

Any set of forces acting on a → rigid body.

→ force; → system.

The condition in a plasma when the → Lorentz force is zero, that is when the electric current flows along the magnetic field. Force-free magnetic fields are encountered in astrophysical plasmas with negligible gas pressure. The solar corona is the best available example of such fields in action in a plasma.

→ force; → free; → magnetic; → field.

1) Physics: Caused by an external force. → forced oscillation.
2) General: Subjected to force, compulsory.

Adjective from → force.

Adjective from zur, → strength.

A fluid flow generated when external forces cause the fluid to flow, for example when a flow is caused by a pump. It contrasts with → free flow.

→ forced; → flow.

The oscillation of a system or object induced by an external periodic force. See also → free oscillation.

→ forced; → oscillation.

Same as the → fundamental interaction.

→ fundamental; → force.

In → Lagrangian dynamics, forces related to → generalized coordinates. For any system with n generalized coordinates q_i (i = 1, ..., n), generalized forces are expressed by F_i = ∂L/∂q_i, where L is the → Lagrangian function.

→ generalized; → force.

The weakest of the four fundamental forces of nature. Described by → Newton's law of gravitation and subsequently by Einstein's → general relativity.

→ gravitational; → force.

Relating to → impulse, the force which is very large but acts for a very short time.

Impulsive, adj. of → impulse; → force.

A force arising from the → acceleration of an observer's → frame of reference.

→ inertial; → force.

A Force acting between molecules.

→ inter-; → molecular; → force.

Within the molecule; occurring by a reaction between different parts of the same molecule.

→ intra-; → molecular.

A metric unit of force which is equal to a mass of one kilogram multiplied by the standard acceleration due to gravity on Earth (9.80665 m sec^-2). Therefore one (1) kilogram-force is equal to 1 kg × 9.80665 m sec^-2 = 9.80665 → newtons.

→ kilogram; → force.

One of many → imaginary lines whose direction at all → points along its length is that of the electric or → magnetic field at those points. In → electric fields the lines of force are directed toward → negative charges and point away from → positive charges. In magnetic fields the lines of force are directed from the → north pole to the → south pole.

→ line; → force.

The force acting upon a → charged particle as it moves in a → magnetic field. It is expressed by F = q.v x B, where q is the → electric charge, v is its → velocity, and B the → magnetic induction of the field. This force is perpendicular both to the velocity of the charge and to the magnetic field. The magnitude of the force is F = qvB sinθ, where θ is the angle between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero. The direction of the force is given by the → right-hand rule.

→ Lorentz; → force.

The force exerted on a spinning object moving through a fluid medium in virtue of → Bernoulli's theorem. The Magnus force can deviate a football from its path when a player strikes it so that it spins about an axis perpendicular to the flow of air around it. As the spinning ball moves through the air, it will create a pressure difference between its two sides. The air travels faster relative to the centre of the ball where its periphery is moving in the same direction as the airflow. This reduces the pressure according Bernoulli's theorem. The opposite effect happens on the other side of the ball, where the air travels slower relative to the centre of the ball. There is therefore an imbalance in the forces that will curve the ball's trajectory.

Named after Heinrich Gustav Magnus (1802-1870), a German chemist and physicist; → force.

A measure of a force's tendency to cause a body to → rotate about a specified → axis. It is given by the force times the perpendicular → distance of the → line of action from the axis. Same as → torque

→ moment; → force.