ALALAL PHY無知一問

在 projectile motion 中, 開頭以 horizontal velocity 飛出去只代表它在飛出的 moment 才是 horizontal.

一開始向下墜後, 它會有 vertical velocity 的 component 而此時 gravitational force 仍然向下, 換言之此時 velocity 和 gravitational force 已經 NOT perpendicular to each other.

所以 projectile motion 中 gravitational force does work on the object, 而 work done 的量就相當於 object's loss in gravitational potential energy (suppose air resistance is negligible)

Note: Gravitational force is also known as conservative force, i.e. the work done by it is independent of the path that the object takes in the journey.

The main difference between the two, projectile motion under gravity and circular motion, is that in the former the (gravitational) force is constantly pointing in one direction (i.e. vertically downward), whereas in the latter the (centripetal) force is constantly changing in direction as the object moves.