Assertion : According to law of conservation of mechanical energy change in potential energy is equal and opposite to the change in kinetic energy.

Reason : Mechanical energy is not a conserved quantity. 

For conservation forces the sum of kinetic and potential energies at any point remains constant throughout the motion,. This is known as law of conservation of mechanical energy. According to this law,

Kinetic energy + Potential energy = Constant

or $∆K + ∆U = 0 or ∆K = -∆U$