Projectile motion is the motion of objects thrown or projected into the air and subjected to only the acceleration due to gravity. The object is called a projectile and the path it follows is its trajectory. In this section, you will learn the theory behind projectile motion and how to calculate the values you need to know about the motion of a projectile.

Key terms

• Uniform motion: Motion at a constant speed.
• Uniformly accelerated motion: Motion with a constant acceleration where the rate of change of velocity is constant.

Projectile motion can be observed in many everyday situations. Throwing a baseball or volleyball, shooting a rifle or an arrow are obvious examples of a projectile. Galileo studied projectiles and found that they moved in two directions at the same time. He determined that the trajectory of a projectile, neglecting air resistance, is a parabolic path (Figure 14).

Parabolic path of a projectile that leaves the origin with velocity Gravity influences the vertical motion of a projectile by accelerating it downward while the horizontal motion is not affected by gravity. So, a projectile undergoes a uniform horizontal motion and vertical uniformly accelerated motion. The horizontal component of acceleration of the projectile, a_x = 0 and the vertical acceleration of the projectile a_y = g = 9.8 m/s².

Thus, when analyzing projectile motion, consider it to be the superposition of two motions: (1) constant-velocity motion in the horizontal direction and (2) free-fall motion in the vertical direction. The horizontal and vertical components of a projectile’s motion are completely independent of each other and can be handled separately, with time t as the common variable for both components.