Identify the INCORRECT pair regarding motion and their examples?

Correct — B. Oscillatory motion is to-and-fro movement about a fixed mean position — a pendulum, a swing, a plucked string. The body has to pass back through the same points in the opposite direction.

The Earth's revolution never does that. It travels around the Sun on a closed orbit and repeats after a fixed interval, which makes it periodic, not oscillatory.

The question asks for the pair that fails, and option (b) is that pair.

Why the others are wrong

• (a)
  Periodic motion – Hands of a clock — A correctly matched pair, so not what the stem wants: the hands of a clock repeat their movement at fixed intervals, which is periodic motion.
• (c)
  Translatory motion – A ball falling from the cliff — Also correct: a ball falling from a cliff moves bodily from one point to another without turning, which is translatory motion (rectilinear here).
• (d)
  Rotatory motion – blades of a fan — Also correct: fan blades turn about a fixed axis through the hub, which is the definition of rotatory motion.

Motion is sorted by the path taken and by what the body does about that path.

Translatory — every part of the body moves the same distance in the same direction, rectilinear on a straight path and curvilinear on a bent one.
Rotatory — the body turns about a fixed axis.
Oscillatory — to-and-fro about a mean position.
Periodic — any motion that repeats at equal intervals.

The last one overlaps the rest: every oscillation is periodic, but not every periodic motion oscillates. The Earth's orbit is the standard counter-example.

Key facts

• Oscillatory motion is to-and-fro motion about a mean position, as in a simple pendulum.
• The Earth's revolution around the Sun is periodic motion, not oscillatory motion.
• Every oscillatory motion is periodic, but not every periodic motion is oscillatory.
• Rotatory motion is turning about a fixed axis, as with fan blades.

Common traps

• Treating repetition as enough for oscillatory, when repetition alone only makes it periodic.
• Marking a correctly matched pair in a question that asks for the incorrect one.

No directly related past PYQ was found.