An object thrown in the air follows a parabolic path under the influence of Earth's gravitational force. The motion of such an object is called projectile motion, and the object itself a projectile. The parabolic path followed by the projectile is called the trajectory. Some common examples of projectile motion are the launching of fireworks, a golf ball in the air, meteors entering the Earth's atmosphere, and the firing of bullets.
When an object falls under gravity and has no horizontal movement, it is called one-dimensional projectile motion. However, when there is horizontal and vertical motion, it is called two-dimensional projectile motion. Motions along the two perpendicular axes are independent and thus can be analyzed separately. Generally, the effect of air resistance is neglected when deriving two-dimensional projectile motion equations.
When analyzing two-dimensional projectile motion, the origin is considered to be the point where the projectile is launched. Since acceleration due to gravity always acts downward, the horizontal component of the projectile's acceleration is always zero throughout the trajectory. Similarly, the vertical component of velocity becomes zero when the projectile reaches the maximum height. Therefore, to solve equations of projectile motion, this choice of axes becomes highly convenient, and hence is widely used.
