Nothing in the world is more common than unsuccessful people with talent.

# Lecture 8 Motion in a straight line

**Example 3.9** Two parallel rail tracks run north-south. Train A moves north with a speed of 54 km h^{–1} with a speed of 90 km h^{–1} , and train B moves south . What is the

(a) velocity of B with respect to A ?,

(b) velocity of ground with respect to B ?, and

(c) velocity of a monkey running on the roof of the train A against its motion (with a velocity of 18 km h^{–1} with respect to the train A) as observed by a man standing on the ground ?

**Exercise 3.1** In which of the following examples of motion, can the body be considered approximately a point object:

(a) a railway carriage moving without jerks between two stations.

(b) a monkey sitting on top of a man cycling smoothly on a circular track.

(c) a spinning cricket ball that turns sharply on hitting the ground.

(d) a tumbling beaker that has slipped off the edge of a table.

**Exercise 3.2** The position-time (x-t) graphs for two children A and B returning from their school O to their homes P and Q respectively are shown in Fig. 3.19. Choose the correct entries in the brackets below ;

(a) (A/B) lives closer to the school than (B/A)

(b) (A/B) starts from the school earlier than (B/A)

(c) (A/B) walks faster than (B/A)

(d) A and B reach home at the (same/different) time

(e) (A/B) overtakes (B/A) on the road (once/twice).

**Exercise 3.3** A woman starts from her home at 9.00 am, walks with a speed of 5 km h^{–1} on a straight road up to her office 2.5 km away, stays at the office up to 5.00 pm, and returns home by an auto with a speed of 25 km h^{–1}. Choose suitable scales and plot the x-t graph of her motion.