An object's motion changes A. only if the object is at rest when a force acts upon it.
B. when there are no forces acting upon it.
C. only when a nonzero net force is applied to the object.
D. when the forces acting on the object remain balanced.

An electron moves with a constant horizontal velocity of 3.0 × 106 m/s and no initial vertical velocity as it enters a deflector inside a tv tube. the electron strikes the screen after traveling 26 cm horizontally and 19 cm vertically upward with no horizontal acceleration. what is the constant vertical acceleration provided by the deflector? (the effects of gravity can be ignored.)