The acceleration of 9.80 m/s^2 occurs in a vacuum. In the Earth’s atmosphere, a feather
and a small lead ball dropped from the same height will not land at the same time
because the feather, with its greater surface area, experiences more air resistance.
Since it has less mass than the ball, gravity exerts less force on it to overcome the
larger air resistance. The acceleration will also be different with two objects of the same
mass but different surface areas: A flat sheet of paper will take longer to reach the
ground than the same sheet crumpled up into a ball.
By convention, “up” is positive, and “down” is negative, like the values on the y axis of a
graph. This means when using g in problems, we state free-fall acceleration as
negative 9.80 m/s^2. To make this distinction, we typically use a or ay when we are
using the negative sign to indicate the direction of free-fall acceleration.
Free-fall acceleration occurs regardless of the direction in which an object is moving.
For example, if you throw a ball straight up in the air, it will slow down, accelerating at
í9.80 m/s^2 until it reaches zero velocity. At that point, it will then begin to fall back
toward the ground and continue to accelerate toward the ground at the same rate. This
means its velocity will become increasingly negative as it moves back toward the
ground.
The two example problems in this section stress these points. For instance, Example 2
on the right asks you to calculate how long it will take a ball thrown up into the air to
reach its zero velocity point (the peak of its motion) and its acceleration at that point.