# space & time

Explorations of multidimensional space and time with linear and angular motion.

## Galilean invariance of the wave equation

This post follows James Rohlf’s Modern Physics from α to Z0 (p.104-105). See also the slides here. Start with the standard configuration for relativity. The (3+1)D space dependent Galilean transformation is The dual (1+3)D time dependent Galilean transformation is The y, z, s, and r axes are trivially invariant. Consider the one-dimensional wave equation for an …

## Rates and inverses

This post is the latest in a series on rates. A rate is a variable quantity measured with respect to a quantity determined independently. A rate is expressed as a ratio of the quantity measured and the independent quantity. A rate of change is a difference of quantities measured with respect to a difference of …

## An ambiguous problem

Here is a simple word problem: a vehicle travels 80 km in 2 hr, then 60 km in 1 hr. What is its average speed? It is ambiguous because the independent variable is not stated or implied. Was the distance measured based on the time, or was the time measured based on the distance? In …

## Rates of change

The difference quotient is the average rate of change of a function between two points: The instantaneous rate of change is the limit of the difference quotient as t1 and t0 approach each other, which is the derivative of f(t) at that point, denoted by f′(t). Derivatives are added by arithmetic addition, i.e., if f(t) …

## Arithmetic of rates

Euclid wrote that “A ratio is a relation in respect of size between two magnitudes of the same kind.” Magnitudes of different kinds were not considered until Galileo. A ratio between magnitudes a and b is expressed as either a : b or b : a. A quotient is the result of dividing a dividend …

## Two kinds of vector rates

This post builds on the previous one here. Vector rates rates of change are of two kinds. An ordinary rate for the vector change of f relative to a unit of x is defined as: The reciprocal vector rate is the vector reciprocal of an ordinary rate with a vector change of g relative to …

## From ratios to quotients

Ratios and proportions are symmetric. A:B ≡ B:A and A:B :: C:D iff C:D :: A:B. But when ratios are converted to quotients or fractions, they are no longer symmetric. There must be a convention as to which is the denominator and which is the numerator. In an ordinary fraction or quotient or rate the …

## Speed, pace and inverses

Legend: x-axis coordinate, x; length interval, Δx; independent length interval, x; t-axis coordinate, t; time interval, Δt; independent time interval, t. For scalars: Rates Speed/Pace Symbols Space Time Ordinary (Time) Speed Δx(t)/t measure given Inverse converse Inverse pace (space speed) x/Δt(x) given measure Converse (Space) Pace Δt(x)/x given measure Inverse Inverse speed (time pace) t/Δx(t) …

## Instantaneous speed and inverse

This post relates to the previous one here. Speed is the time rate of distance traversed. Pace is the space rate of elapsed time. The (time) speed of a body is the distance traversed per unit of independent time without regard to direction, Δx/t. The instantaneous speed is the speed at a point in space …