The force $\mathbf{F}$ experienced by a charge $q$ in an electric field $\mathbf{E}$ is given by $\mathbf{F} = q\mathbf{E}$ . For an electron, the charge is negative ($q = -e$), which implies that the electrostatic force acts in the direction opposite to the electric field vector . The problem states that the electron enters with a velocity $\mathbf{v}$ in the direction of the electric field lines. Therefore, the force $\mathbf{F}$ acts opposite to the direction of velocity. This opposing force acts as a retarding force, causing deceleration, which results in a decrease in the velocity of the electron. The path remains a straight line because the force and velocity are collinear.