Question

What is common to the following?
Motion of the propeller of a flying helicopter, the minute hand of a watch, the tape of the cassette recorder
A. All are examples of translatory motion
B. All are examples of oscillatory motion
C. All are examples of rotatory motion
D. All are examples of periodic motion

Answer 1

For this question, we shall first describe the motion of a propeller, the minute had and the tape of the cassette recorder. The propeller undergoes a circular motion where the blades of the propeller move radially about a fixed circle.Similarly, the minute hand in a watch moves radially around the watch and the tape of the cassette recorder moves radially about the segment of the recorder in which the tape is placed.So, in all these motions, the movement is circular and radial. This would mean that the object would cover equal distances in equal intervals of time. Also, this motion is repeated in equal intervals of time.

Complete answer:
Translatory motion is motion characterized along a straight-line path or a curved path with a constant acceleration. In such a motion all particles of the object experience the same force.An oscillatory motion is the to and fro motion of any object about an equilibrium point under some restoring force.

A rotatory motion is characterized by the rotation of a rigid body about an axis. In this kind of motion, all particles in the rigid body experience different torques depending upon the distance from the chosen axis.In a periodic motion, the body repeats its motion after a definite time interval. This time interval is known as its time period.

In the given question, the motions are such that the objects move in a circular path and repeat their motion after a definite time interval. Hence, they all undergo periodic motion.

Hence option D is correct.

Note: Keep a note of the difference between the periodic motion and the oscillatory motion. An oscillatory motion is a to and fro motion about one point. However periodic motion is any kind of motion that repeats itself after a definite time interval. Thus, we can say that oscillatory motion is a subset of the periodic motions.