Why does work equal kinetic energy?

Relation bewteen KE and W: The work done on an object by a net force equals the change in kinetic energy of the object: W = KEf - KEi. This relationship is called the work-energy theorem. read more

Kinetic Energy and the Work Energy Theorem Idea: Force is a vector, work and energy are scalars. Thus, it is often easier to solve problems using energy considerations instead of using Newton's laws (i.e. it is easier to work with scalars than vectors). read more

Friction basically produces heat energy. In order to get this heat energy we need to take it from somewhere else, like kinetic energy. Energy is the ability to do work so in your problem the work done by friction is equal to the loss of kinetic energy of the baseball player. read more