We need one more thing—how about Newton's second law? This says the acceleration depends on the net force (F_net) and the mass (m) of an object. It’s usually written as F_net = m × a, but we can rearrange it like this: a = F_net/m. Combining this with our gravitational force, we get something pretty interesting:

Since both gravity and acceleration depend on the mass of the ball, the mass cancels. We find that any 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 marble at the same time, they’ll hit the ground at the same time—even though the gravitational force on the bowling ball is thousands of times higher. Weird, right?

Anyway, now, in the presence of gravity, if you kicked a ball at an upward angle, it’s vertical velocity would slow, halt, and reverse, with the speed increasing as it falls. In other words, it starts accelerating in the downward direction as soon as it’s kicked, even while it’s moving upward.

What about the horizontal motion? Ah, since there’s no horizontal force after the initial kick, the ball continues traveling forward at the same speed, just like in space. People tend to think a ball falls because its forward motion slows, but actually it’s the opposite. Without air drag it doesn’t slow down at a...