Q. A small sphere of mass $m$ is dropped from a great height. After it has fallen $100\ m$, it has attained its terminal velocity and continues to fall at that speed. The work done by air friction against the sphere during the first $100\ m$ of fall is
A) Greater than the work done by air friction in the second $100\ m$
B) Less than the work done by air friction in the second $100\ m$
C) Equal to $100\ mg$
D) Greater than $100\ mg$
Explanation
(b) In the first $100\ m$ body starts from rest and its velocity goes on increasing and after $100\ m$ it acquire maximum velocity (terminal velocity). Further, air friction $i.e.\ $viscous force which is proportional to velocity is low in the beginning and maximum at $v = v_{T}$
Hence work done against air friction in the first $100\ m$ is less than the work done in next $100\ m$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.