NCERT Class 11 Physics Solutions: Chapter 3 – Motion in a Straight Line Part 1

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Question: 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.

Answer:

(a) The size of a carriage is very small as compared to the distance between two stations. Therefore, the carriage can be treated as a point-sized object.

(b) The size of a monkey is very small as compared to the size of a circular track. Therefore, the monkey can be considered as a point sized object on the track.

(c) The size of a spinning cricket ball is comparable to the distance through which it turns sharply on hitting the ground. Hence, the cricket ball cannot be considered as a point object.

(d) The size of a beaker is comparable to the height of the table from which it slipped. Hence, the beaker cannot be considered as a point object.

Question: 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 figure. 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).

Figure shows the position - time (x-t) graphs

Figure Shows the Position - Time (X-T) Graphs

Figure shows the position - time (x-t) graphs

Answer:

(a) A live closer to school than B.

(b) A starts from school earlier than B.

(c) B walks faster than A.

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

(e) B overtakes A once on the road.

Explanation:

(a) In the given X – t graph, it can be observed that distance Hence, the distance of school from the A’s home is less than that from B’s home.

(b) In the given graph, it can be observed that for for A, whereas for has some finite value for B. Thus, A starts his journey from school earlier than B.

(c) In the given graph, it can be observed that the slope of B is greater than that of A. Since the slope of the graph gives the speed, a greater slope means that the speed of B is greater than the speed A.

(d) It is clear from the given graph that both A and B reach their respective homes at the same time.

(e) B moves later than A and his/her speed is greater than that of A. From the graph, it is clear that B overtakes A only once on the road.