Misplaced Pages

#958041

17-620: Relative may refer to: Relative is a term used in physics, and especially in Galilean , special and general relativity , to denote that something is dependent on a reference frame, or that it is taken specifically in a given reference frame ("its velocity relative to the cow is 15.5m/s", "time and Space are relative, not fixed") Galilean relativity Specifically, the term Galilean invariance today usually refers to this principle as applied to Newtonian mechanics , that is, Newton's laws of motion hold in all frames related to one another by

34-540: A Galilean transformation . In other words, all frames related to one another by such a transformation are inertial (meaning, Newton's equation of motion is valid in these frames). In this context it is sometimes called Newtonian relativity . Among the axioms from Newton's theory are: Galilean relativity can be shown as follows. Consider two inertial frames S and S' . A physical event in S will have position coordinates r = ( x , y , z ) and time t in S , and r' = ( x' , y' , z' ) and time t' in S' . By

51-417: A good model, extends to roughly 10 light years. In special relativity, one considers Einstein's cabins , cabins that fall freely in a gravitational field. According to Einstein's thought experiment, a man in such a cabin experiences (to a good approximation) no gravity and therefore the cabin is an approximate inertial frame. However, one has to assume that the size of the cabin is sufficiently small so that

68-415: Is free charge density, P {\displaystyle \mathbf {P} } is polarization density. The magnetic field and free current density are transformed under this transformation when changing frames of reference, but the electric field and related quantities are unchanged Because the distance covered while applying a force to an object depends on the inertial frame of reference, so depends

85-430: Is free current density, M {\displaystyle \mathbf {M} } is magnetization density. The electric field is transformed under this transformation when changing frames of reference, but the magnetic field and related quantities are unchanged. An example of this situation is a wire is moving in a magnetic field such as would occur in an ordinary generator or motor. The transformed electric field in

102-496: Is not consistent if T { ∗ , v 1 + v 2 } ≠ T { ∗ , v 1 } + T { ∗ , v 2 } {\displaystyle T\{*,v_{1}+v_{2}\}\neq T\{*,v_{1}\}+T\{*,v_{2}\}} where v 1 {\displaystyle v_{1}} and v 2 {\displaystyle v_{2}} are velocities. A consistent transformation will produce

119-462: The work done. Due to Newton's law of reciprocal actions there is a reaction force; it does work depending on the inertial frame of reference in an opposite way. The total work done is independent of the inertial frame of reference. Correspondingly the kinetic energy of an object, and even the change in this energy due to a change in velocity, depends on the inertial frame of reference. The total kinetic energy of an isolated system also depends on

136-413: The assumptions and properties of Newton's theory are: In comparison, the corresponding statements from special relativity are as follows: Both theories assume the existence of inertial frames. In practice, the size of the frames in which they remain valid differ greatly, depending on gravitational tidal forces. In the appropriate context, a local Newtonian inertial frame , where Newton's theory remains

153-516: The convergence of gravitational fields in the universe dictates the scale at which one might consider such (local) inertial frames. For example, a spaceship falling into a black hole or neutron star would (at a certain distance) be subjected to tidal forces strong enough to crush it in width and tear it apart in length. In comparison, however, such forces might only be uncomfortable for the astronauts inside (compressing their joints, making it difficult to extend their limbs in any direction perpendicular to

170-1091: The electric field in the initial frame of reference is insignificant, but the magnetic field is strong. When the magnetic field is dominant and the relative velocity, v r {\displaystyle v^{\mathbf {r} }} , is low, then the following transformation may be useful: H ′ = H J f ′ = J f B ′ = B M ′ = M E ′ = E + v r × B {\displaystyle {\begin{aligned}\mathbf {H^{'}} &=\mathbf {H} \\\mathbf {J_{f}^{'}} &=\mathbf {J_{f}} \\\mathbf {B^{'}} &=\mathbf {B} \\\mathbf {M^{'}} &=\mathbf {M} \\\mathbf {E^{'}} &=\mathbf {E} +v^{\mathbf {r} }\times \mathbf {B} \\\end{aligned}}} where J f {\displaystyle \mathbf {J_{f}} }

187-471: The gravitational field is approximately parallel in its interior. This can greatly reduce the sizes of such approximate frames, in comparison to Newtonian frames. For example, an artificial satellite orbiting the Earth can be viewed as a cabin. However, reasonably sensitive instruments could detect "microgravity" in such a situation because the "lines of force" of the Earth's gravitational field converge. In general,

SECTION 10

#1732855860959

204-499: The gravity field of the star). Reducing the scale further, the forces at that distance might have almost no effects at all on a mouse. This illustrates the idea that all freely falling frames are locally inertial (acceleration and gravity-free) if the scale is chosen correctly. There are two consistent Galilean transformations that may be used with electromagnetic fields in certain situations. A transformation T { ∗ , v } {\displaystyle T\{*,v\}}

221-400: The inertial frame of reference: it is the sum of the total kinetic energy in a center-of-momentum frame and the kinetic energy the total mass would have if it were concentrated in the center of mass . Due to the conservation of momentum the latter does not change with time, so changes with time of the total kinetic energy do not depend on the inertial frame of reference. By contrast, while

238-1473: The moving frame of reference could induce current in the wire. Electric field systems are those systems in which the magnetic field in the initial frame of reference is insignificant, but the electric field is strong. When the electric field is dominant and the relative velocity, v r {\displaystyle v^{r}} , is low, then the following transformation may be useful: E ′ = E D ′ = D ρ f ′ = ρ f P ′ = P H ′ = H − v r × D J f ′ = J f − ρ f v r {\displaystyle {\begin{aligned}\mathbf {E^{'}} &=\mathbf {E} \\\mathbf {D^{'}} &=\mathbf {D} \\\mathbf {\rho _{f}^{'}} &=\mathbf {\rho _{f}} \\\mathbf {P^{'}} &=\mathbf {P} \\\mathbf {H^{'}} &=\mathbf {H} -v^{\mathbf {r} }\times \mathbf {D} \\\mathbf {J_{f}^{'}} &=\mathbf {J_{f}} -\rho _{\mathbf {f} }v^{\mathbf {r} }\\\end{aligned}}} where ρ f {\displaystyle \rho _{\mathbf {f} }}

255-401: The same results when transforming to a new velocity in one step or multiple steps. It is not possible to have a consistent Galilean transformation that transforms both the magnetic and electric fields. There are useful consistent Galilean transformations that may be applied whenever either the magnetic field or the electric field is dominant. Magnetic field systems are those systems in which

272-412: The second axiom above, one can synchronize the clock in the two frames and assume t = t' . Suppose S' is in relative uniform motion to S with velocity v . Consider a point object whose position is given by functions r' ( t ) in S' and r ( t ) in S . We see that The velocity of the particle is given by the time derivative of the position: Another differentiation gives the acceleration in

289-421: The two frames: It is this simple but crucial result that implies Galilean relativity. Assuming that mass is invariant in all inertial frames, the above equation shows Newton's laws of mechanics, if valid in one frame, must hold for all frames. But it is assumed to hold in absolute space, therefore Galilean relativity holds. A comparison can be made between Newtonian relativity and special relativity . Some of

#958041