How Can Soccer Players Take Their Kicks in Midair?

We need one more thing—what about Newton’s the second the law? This says that the acceleration depends on the net force (F_{the net}) and plural (m) of something. It is usually written as F_{the net} = m × abut we can rearrange it like this: a = F_{the net}/m. Combining this with our gravity, we get something very interesting:

Courtesy of Rhett Allain

Since gravity and acceleration depend on the mass of the ball, mass cancels. We see that anything on Earth has a speed of less than 9.8 meters per second per second (m/s).²) This means that if you drop a bowling ball and a marble at the same time, they will hit the ground at the same time—even though the gravitational force on the bowling ball is thousands of times higher. Amazing, right?

However, now, in the face of gravity, if you hit the ball at a high angle, its vertical speed would slow down, stop, and bounce back, with speed increasing as it falls. In other words, it starts accelerating downward as soon as it is kicked, even when it is moving up.

What about horizontal motion? Oh, since there is no horizontal force after the initial kick, the ball continues to move forward at the same speed, just like in the air. People tend to think that the ball is falling because its forward momentum is slowing down, but it is actually the opposite. Without inhalation it does not reduce at all. It only stops because the ground gets in the way.

So what we get in sequence is the familiar upside-down parabola, often called the ballistic trajectory because it’s the path of any passive projectile, such as a cannonball, bullet, or basketball. Any flying object on which gravity is the only (significant) force acting on it will move like this.

Football And Air

Fortunately, Earth has air. But it really changes the game. Now there it is a continuous force acting horizontally, which we call air resistance, or drag, and pushes in the opposite direction to the ball’s motion.

Think of air molecules as a bunch of tiny ping-pong balls. As the soccer ball moves through the air it collides with gazillions of these tiny balls of air, and each collision produces a force of push back; all together, this makes up the total air resistance. The bigger the object, the more conflicts it faces.