Why is it difficult to use the law of conservation of energy to calculate the

effects of a collision?

The law of conservation of energy is difficult to use for calculate the

effects of a collision because of energy loss

Explanation:

We know that Collisions are of two types which is elastic and inelastic. The Law of Conservation of energy can be defined as in isolated system the total energy is said to remain constant and stays conserved over time can be applied on elastic collisions only as they don't lose energy and can not be applied on inelastic collisions due to energy loss in them due to heat and mechanical vibration. Hence, law of conservation of energy can not be used to calculate effects of collision

Inelastic collisions actually do conserve energy, but the loss of energy to heat and mechanical vibration is hard to calculate so the math equating energy before and energy after is hard to balance.