Projectile motion is a physical phenomenon that occurs when an object is projected by a force that stops exerting influence on the object after it has been launched. The laws of physics cause objects to follow a very particular path when they are launched in this way. A classic example is a soccer ball, which becomes a projectile when it is kicked by a player.
Most people are familiar with Isaac Newton's statement that an object in motion tends to remain in motion. This is known as inertia. Inertia plays an important role in projectile motion, because it explains why an object keeps moving without any source of propulsion. There is also a force at play: gravity. Gravity explains why the soccer ball in the above example returns to Earth, rather than moving continuously through the air.
The path of a projectile is roughly parabolic. When it is launched, the inertia helps it move upwards, against gravity, but eventually gravity's pull becomes too strong, and the object starts to loop back to Earth. The object has also been traveling horizontally, however, so the path of the object creates an arc. Eventually, the object will hit the Earth and come to rest, and in the case of the soccer ball, to be kicked by another player.
Understanding how projectile motion works is important. Historically, many armies struggled with the concept, because they didn't understand how arrows, cannonballs, bullets, and other projectiles moved through the air, and this made it difficult to aim properly. The physics behind this type of motion also plays a role in sports and many other activities, which is why questions involving this concept ask people to use mathematical formulas to determine the path that a projectile will follow occur on so many physics tests.
The initial velocity of the object, mass, and angle of launch all play a role in the path that the object will follow; for example, a marble that rolls off a table will follow a different route than one that is launched upwards with a small catapult. Most projectile motion problems are set on Earth, which has a familiar gravity, although people can also calculate it for various objects on other planets, as long as the gravity is known. Simple problems also assume that air resistance and the Earth's rotation are unimportant, although in fact they can become issues with certain types of problems.