Draw a curved line CD. Take a thread and make a knot at one of its ends. Keep the knot made in the thread on point C of the line. Place a small portion of the thread along the line, keeping it tight using your fingers and thumb. Keep on stretching the thread on the curved line till you take the thread to the point D of the line. Make a mark on the thread where it touches the other end of the line at point D. Now, stretch the thread along a metre scale and the length between the knot in the beginning and mark on the thread is measured through scale. This measured length on scale will give you the length of the curved line CD.

(a) Rectilinear motion The handle of bicycle will always move in rectilinear path because it cannot do circular or rotatory motion.
(b) Circular motion The Paddles of bicycle will always move circularly around its chain fixing system because it cannot move in forward direction without the whole chain system.
(c) Both rectilinear and circular The wheels of bicycle will move in rectilinear as well as in circular motion because the wheel as a whole will move forward and its point or particles around the rim will execute circular motion.

Elastic tapes are stretchable due to which their length can be increased during the measurement if it is not handle with care. Therefore, the measurement done by elastic tapes may be greater than the actual length of the object. So, we can say that elastic tapes are not accurate to measure a length.

The reasons may be as follows
(i) Their scales of measurement may not be standard or they may be using different scales of measurement.
(ii) The length of the scale may not be proper, i.e. the length of scale may be shorter ·than the length they want to measure.
(iii) There may be some errors in the scale which they are using or they may not be using the correct method of observing the scale.

The motions of a wheel of a moving bicycle and a mark on the blade of a moving electric fan are different in the following ways.

While travelling, trees near the track same to be moving back (i.e. opposite to the direction of motion of a train) because there is a relative motion between outside trees and moving train. While in case of co-passengers, the relative motion between we and co-passengers, is zero, so it appears to be stationary.