A pseudo force (or fictitious force) is introduced when analyzing motion from the perspective of a non-inertial (accelerating) reference frame.

• If the frame is accelerating with ${\vec{a}}_{frame}$ relative to an inertial frame, an object of mass $m$ experiences a pseudo force ${\vec{F}}_p=-m{\vec{a}}_{frame}$.
• The negative sign indicates the pseudo force is always directed opposite to the acceleration of the frame.
• Since the frame moves right, the pseudo force acts to the left with magnitude $ma$.