Question

A stone of mass $m$ kg is whirled in a vertical circle of radius $20$ cm. The difference in the kinetic energies at the lowest and the topmost position is:

• A. $4mgjoule$
• B. $0.4mgjoule$
• C. $40mgjoule$
• D. None of these

Answer 1

The correct option is B $0.4mgjoule$

According to the law of conservation of energy
Total energy of the system (KE $+$ PE) will be equal at both , topmost and bottom most point.
Therefore, $△PE=△KE$
$K{E}_b-K{E}_t=P{E}_t-P{E}_b$
$P{E}_t-P{E}_b=m\times g\times (2\times r)$
$△KE=0.4mg$ $joule$