If a ball is thrown up in the air, not vertically and not horizontally but at an angle to the horizontal, then it follows a path of the formAny path of this form is called a parabola. The ball, a projectile, describes a parabola, and this is true for all projectiles.

The only force acting on a projectile is the force of gravity. Air resistance is ignored. Gravity is a vertical force, acting downwards,. There are no horizontal forces therefore, and no horizontal acceleration. The horizontal velocity, v-from the diagram below.

The projectile accelerates downwards with an acceleration g. We can use the equation of motionvertically to obtain the vertical component of the velocity at any timeWithwe haveThis means that v decreases as t increases, becoming zero at the highest point and negative thereafter.