As a projectile moves along its parabolic trajectory, which of the following remain constant (ignoring air resistance, and defining the z-axis to point upward)? More than one answer may be correct! a. Its speed. b. Its velocity. c. Its x-velocity and its y-velocity. d. Its acceleration I am thinking the only thing constant is acceleration. My book keeps saying that the horizontal velocity is constant but this is a parabolic trajectory so shouldn't that make a difference? Also velocity is going to change from positive to negative, and speed will change at different points of the trajectory.