Translate item 2 responses in English

Projectile motion is the motion of an object thrown or projected into the air, subject only to the force of gravity. The object follows a curved path called a trajectory.

The work-energy theorem states that the net work done on an object is equal to the change in its kinetic energy.

When applied to motion under gravity and non-conservative forces (such as air resistance), the theorem states that the work done by non-conservative forces ($W_{nc}$) is equal to the change in the total mechanical energy (kinetic energy plus gravitational potential energy).

Where $K$ is kinetic energy and $U_g$ is gravitational potential energy.