We need one more thing – maybe Newton second law? This means that the acceleration depends on the resultant force (FInternet) and mass (M) object. This is usually written as FInternet = m × abut we can arrange it as follows: a = FInternet/M. Combining this with our gravitational force, we get something quite compelling:
Courtesy of Rhett Allain
Since both gravity and acceleration depend on the mass of the ball, this mass decays. We found that every object on Earth has a downward acceleration of 9.8 meters per second per second (m/s2). This means that if you drop a bowling ball and a ball at the same time, they will hit the ground at the same time – even though the force of gravity on the bowling ball is thousands of times greater. Strange, right?
Anyway, now, in the presence of gravity, if you kick a ball at an upward angle, its vertical speed will sluggish down, stop, and invert, with the speed increasing as it falls. In other words, it starts accelerating downwards as soon as it is kicked, even when it is moving upwards.
What about horizontal movement? Ah, since there is no horizontal force after the first kick, the ball flies forward at the same speed, just like in space. People often think that the ball is falling because its forward motion is slower, but in fact the opposite is true. Without air resistance it doesn’t sluggish down at all. It only stops because the ground is in the way.
So the trajectory is the familiar inverted parabola, often called a ballistic trajectory because it is the path of any unpropelled projectile, such as a cannonball, bullet, or basketball. Any flying object for which gravity is the only (significant) force acting on it will move in this way.
Football In The Air
Fortunately, there is air on Earth. But this changes the game drastically. Now there Is a continuous force acting horizontally, which we call air resistance or drag, and pushes in the direction opposite to the motion of the ball.
Think of air molecules as a bunch of petite ping-pong balls. As the ball moves through the air, it collides with billions of these little air balls, and each collision exerts a backward repulsive force; all of this together creates the total air drag force. The larger the object, the more collisions it has to overcome.