Examples objects moving at constant speed but whose velocity is not constant?

Answer 1

ANSWER 1: Consider any solid comprising a lattice of molecules, moving at constant speed, say in the vacuum of space. The electrons within the atoms of the molecules are subatomic "objects" whose position and instantaneous velocity are best described by a probability density field equation, whose surface lies within the lattice of atoms within the solid. At any instant in time, the position and velocity of the electrons is not a constant, whereas the "speed" of the electrons which is their velocity averaged over say one second is the same as that of the solid comprising the lattice of molecules.

ANSWER 2: Consider a "photon" of light which may be regarded as an object (since it is a fundamental particle). As we all know the speed of light is a constant = C. However when passing near a massive object (eg black hole) the photon travels along the shortest path of the curvature of space-time in the vicinity of the massive object. Relative to the photon, its speed is and always has been = C. However when the speed of the photon is measured by a distant observer, it appears that for the period while under the influence of the curvature in space-time near the massive object, the velocity of the photon has varied. Hence the apparent shift in position that occurs when stars are observed in the vicinity of a black hole. In the limit, at the event horizon of a black hole, photons are apparently "slowed" to the extent that their velocity is no longer measurable by the remote observer, hence the inside of a black hole is "black", devoid of light.

These answers provided by Rob Sherratt 18th July 2009

Answer 2

An object moving in a circle at a constant speed has a changing velocity because its direction is continuously changing. Similarly, an object moving on an inclined plane at a constant speed will have a changing velocity due to the gravitational force affecting its motion.