Motion in a Straight Line

This question belongs to the Motion in a straight line chapter that we will be solving this with explanation.

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

Whenever a railway carriage moves between two stations smoothly without jerks, it is considered as a point object. This is due to the fact that distance between the two stations is larger as compared to the size of carriage. However, we are interested in the overall motion instead of the movement of specific parts of carriage.

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

Here, both monkey as well as the cyclist are considered as a single point object if we analyze their motion on the circular track. The radius of the circular path is much larger as compared to the size of the monkey and cyclist. This makes the point object approximation valid for studying overall motion.

There are more complex and other important questions in the NCERT excercise of Motion in a Straight Line that you can check out for verifying if you are answering questions correctly. 

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

A spinning cricket ball that turns sharply on hitting the ground is not a point object. The spin and sharp turn are highly influenced by the size, shape and rotation of the ball. This affects the trajectory of the ball and its interaction with the ground.

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

A tumbling beaker which has slipped off a table is not a point object since it involves rotation and changes in the orientation. These depend on the shape and size of beaker. 

Students must go through those solutions if they are currently studying in class 12th and soon they have to take the 12th CBSE board exam.