 All movement on Earth is regulated by the law of gravity. It's a natural phenomenon by which physical bodies attract each other. For example, a rock is attracted to the Earth. However, because the Earth is incredibly larger than a rock, indeed the Earth remains fixed, but the rock is strongly attracted to the Earth. From our Earth-bound perspective, the rock is falling to Earth. Due to Earth's gravitational pull, the free-falling rock is accelerating towards Earth. This gravitational force is experienced as an acceleration of 9.8 meters per second per second, or 9.8 meters per second squared. However, for simplicity, gravitational acceleration is often rounded to 10 meters per second squared. To illustrate the effects of gravity on free-falling objects, let's look at what happens when you drop a rock from a cliff. With no wind resistance, the rock's velocity increases by 10 meters per second for every second it falls. Assuming the rock is falling from rest, after one second its velocity is 10 meters per second. After two seconds, its velocity is 20 meters per second. After three seconds, it is falling at a velocity of 30 meters per second. With each passing second, the rock is gaining speed by 10 meters per second until it's stopped by the surface of the Earth. Now, let's look at how gravity interacts with an object that is thrown upward away from the Earth and against gravitational pull. How will gravity affect an object's velocity at various points along its airborne pathway? In this illustration, a baseball is thrown straight up into the air. The ball leaves the hand of the person with an initial velocity of 30 meters per second. Even though the ball's initial trajectory opposes gravity, it's experiencing the influence of gravitational pull throughout its flight. As the ball travels upward away from the Earth, it slows down or decelerates at a rate of 10 meters per second squared. After one second, the ball is only traveling upward at 20 meters per second. Two seconds later, its velocity is 10 meters per second. After three seconds, the ball is traveling at zero meters per second. At the height of the ball's trajectory before it succumbs to the pull of gravity and descends back to Earth, the baseball has a velocity of zero. This moment of zero velocity is known as a state of equilibrium. Immediately, the ball begins to free fall from this momentary stationary position and experiences an acceleration back toward the Earth at 10 meters per second per second. After falling for three seconds, the baseball is at a velocity equal to its initial velocity of 30 meters per second.