The kinetic energy of a particle increases continuously over time.

The kinetic energy of a particle is given by $K = \dfrac{p^2}{2m}$.

Since the kinetic energy increases continuously, the magnitude of its linear momentum $p$ must also increase continuously.

The rate of change of kinetic energy is equal to the power delivered by the net force, which is given by $\dfrac{dK}{dt} = \vec{F} \cdot \vec{v} = F v \cos \theta$.

For the kinetic energy to increase continuously, $\dfrac{dK}{dt} > 0$, which implies $\cos \theta > 0$.

Thus, the angle $\theta$ between the resultant force and the velocity must always be less than 90^{}^{{}^{\circ}}.

The height of the particle does not necessarily decrease, as an upward force greater than gravity can increase both its height and kinetic energy.

The resultant force need not be strictly parallel to the velocity; it only requires a positive tangential component.