CBSE Class 11 Physics Chapter 3: Motion in a Straight Line NCERT Solutions

An object can be considered a point object if its size is negligible compared to the distances it travels or the scale of the phenomenon being observed. Let's analyze each case:

1. A railway carriage moving without jerks between two stations: The distance between two stations is typically very large compared to the dimensions of a railway carriage. Therefore, the carriage can be reasonably approximated as a point object for this motion.
2. A monkey sitting on top of a man cycling smoothly on a circular track: The radius of a circular track is usually much larger than the size of a monkey or a person. Thus, the monkey can be treated as a point object relative to the track's dimensions.
3. A spinning cricket ball that turns sharply on hitting the ground: The distance over which a cricket ball turns sharply after hitting the ground is comparable to the size of the ball itself. Therefore, the ball cannot be considered a point object in this scenario, as its rotation and spin are significant.
4. A tumbling beaker that has slipped off the edge of a table: The distance the beaker falls is often comparable to its own dimensions, especially if it's a short fall. Thus, it cannot be accurately represented as a point object.

Based on this analysis, the correct options are (a) and (b).

Let's analyze the provided position-time (x-t) graphs to determine the correct statements.

1. (A) lives closer to the school than (B).

   The graph shows the position (x) versus time (t). The school is at the origin (x=0). The final position of child A (home P) is closer to the origin than the final position of child B (home Q). Therefore, A lives closer to the school.

2. (A) starts from the school earlier than (B).

   The graph indicates that child A's journey starts at time t=0 (the graph begins at the origin). Child B's journey starts at a later time, as their graph begins at a positive value of t when x=0. Thus, A starts earlier.

3. (B) walks faster than (A).

   The speed of an object is represented by the slope of its position-time graph. Observing the graph, the slope of B's line is steeper than the slope of A's line. A steeper slope signifies a higher speed. Therefore, B walks faster than A.

4. A and B reach home at the same time.

   Both graphs end at the same point on the time axis. This indicates that both children reach their respective homes at the same final time, even though they started at different times and traveled at different speeds.

5. (B) overtakes (A) on the road once.

   An overtaking event occurs when the object that started later or was moving slower is overtaken by the object that started earlier or is moving faster, resulting in them being at the same position at the same time. In this graph, B starts later but moves faster. The point where B's graph intersects A's graph represents the moment B catches up to and overtakes A. This intersection occurs only once.