a transformer is used to increase or decrease a voltage. but it has to be an ac voltage, or the transformer doesn't work.
using the law of conservation of energy
when two cars collide with each other then the momentum is same before collision and after collision but energy is changed after collision in form of heat and sound.
we know this collision is inelastic collision.
in inelastic collision, when two objects collide with each other then the momentum is conserved but kinetic energy is not conserved.