Aball is thrown straight upward so that it goes up and then comes back down. on the way up, the ball slow down. on the way down, the ball speeds up. does the direction of the acceleration vector change during this process?

Some type of thing that was invisible

The answer is frictional force

stands for electromotive force and it is measured in volts. despite its name, it is not actually the force in the sense of being a "pushing" or "pulling" (not measured in newtons).an equation that relates emf,  ℰ,  to a battery's terminal voltage, v, current, i, and internal resistance, r, is    ℰ = v + irin this case, the emf can be interpreted as a voltage—a difference in electric potential.i would not say that it would be considered an energy. if emf can be defined as the energy/work per a unit charge from a battery or other electric source, then it is not strictly energy.