Unit I: Free Fall
The Universal Acceleration of Gravity.
This is the final chapter of Unit I. It applies our knowledge of acceleration to a real-world force.
← Go back to Chapter 2: Acceleration.
The Problem: Do Heavier Objects Fall Faster?
For nearly 2,000 years, the teachings of the Greek philosopher Aristotle were accepted as fact. He argued that heavier objects fall faster than lighter ones. It seems intuitive, right? A bowling ball will certainly hit the ground before a feather.
But in the 16th century, a scientist named Galileo Galilei began to question this. He noticed that two stones of different weights, dropped from a height, seemed to land at almost the exact same time. What was really going on?
The Leaning Tower of Pisa Experiment
Legend has it that Galileo dropped two objects of different masses from the top of the Leaning Tower of Pisa to prove his theory. Use the simulator below to recreate this famous experiment.
The "Aha!" Moment
With air resistance turned off (in a vacuum), both the bowling ball and the feather hit the ground at the exact same time! This proves Galileo's theory: an object's mass does not affect its acceleration due to gravity.
The only reason the feather falls slower in real life is because of air resistance, which has a much greater effect on light, spread-out objects. On the moon, which has no air, a hammer and a feather dropped by Apollo 15 astronauts landed simultaneously.
The Acceleration Due to Gravity (g)
Galileo's discovery was profound. He found that, ignoring air resistance, all objects in free fall accelerate downwards at the same constant rate. We call this the acceleration due to gravity, represented by the letter g.
This means that for every second an object is falling, its downward velocity increases by about 9.8 meters per second. This is a special case of the constant acceleration we studied in the last chapter. All of our kinematic equations still apply—we just substitute -9.8 m/s² for the acceleration `a` (it's negative because it's in the downward direction).
Congratulations! You've Completed Unit I.
You have now mastered Kinematics, the science of *describing* motion. The next big question is: *why* do objects move and accelerate in the first place? This brings us to the world of forces and Newton's Laws.