Kinetic energy of an object of mass, moving with a velocity, v is given by the expression,
E k= 1/2 mv2
Explanation
In order to bring it to rest, its velocity has to be decreased to zero, and in order to achieve that, the kinetic energy has to be drained off and sent somewhere else. An external force has to absorb energy from the object, i.e. do negative work on it, equal to its kinetic energy,or -1/2 mv2
Good explaination. Doubt cleared. Thank you.