It results change only in it's kinetic energy, it's KE will increase in accord with the work-energy theorem
Look at the first person’s answer. Cause I know I’m wrong
Answer:

Explanation:
The motion of ballistic pendulum is modelled by the appropriate use of the Principle of Energy Conservation:

The final velocity of the system formed by the ballistic pendulum and the bullet is:



Initial velocity of the bullet can be calculated from the expression derived of the Principle of Momentum:

