1. The Acceleration Due to Gravity

2. FREE FALLING
a free-falling object is an object that is falling under the sole influence of gravity having an acceleration of 9.8 m/s/s, downward (on Earth). This numerical value for the acceleration of a free-falling object is such an important value that it is given a special name. It is known as the acceleration of gravity - the acceleration for any object moving under the sole influence of gravity.

3. What is g?
the acceleration of gravity is such an important quantity that physicists have a special symbol to denote it - the symbol g
g = 9.8 m/s/s, downward

4. How Fast? and How Far?
Free-falling objects are in a state of acceleration. Specifically, they are accelerating at a rate of 9.8 m/s/s. This is to say that the velocity of a free-falling object is changing by 9.8 m/s every second. If dropped from a position of rest, the object will be traveling 9.8 m/s (approximately 10 m/s) at the end of the first second, 19.6 m/s (approximately 20 m/s) at the end of the second second, 29.4 m/s (approximately 30 m/s) at the end of the third second, etc.

6. The Big Misconception
Questions are often asked "doesn't a more massive object accelerate at a greater rate than a less massive object?" "Wouldn't an elephant free-fall faster than a mouse?" This question is a reasonable inquiry that is probably based in part upon personal observations made of falling objects in the physical world.
Watch: The Big Misconception

7. So what’s the answer..?
Acceleration of an object is directly proportional to force and inversely proportional to mass. Increasing force tends to increase acceleration while increasing mass tends to decrease acceleration. Thus, the greater force on more massive objects is offset by the inverse influence of greater mass. Subsequently, all objects free fall at the same rate of acceleration, regardless of their mass.

8. Think about it!
Suppose that an elephant and a feather are dropped off a very tall building from the same height at the same time. Suppose also that air resistance could somehow be eliminated such that neither the elephant nor the feather would experience any air drag during the course of their fall. Which object - the elephant or the feather - will hit the ground first?

9. Example: Let’s watch an elephant and a mouse dropping from a building!

10. True or False?
The elephant and the feather each have the same force of gravity.
The elephant has more mass, yet both elephant and feather experience the same force of gravity.
The elephant experiences a greater force of gravity, yet both the elephant and the feather have the same mass.

11. True or False?
On earth, all objects (whether an elephant or a feather) have the same force of gravity.
The elephant weighs more than the feather, yet they each have the same mass.
The elephant clearly has more mass than the feather, yet they each weigh the same.

12. True or False?
The elephant clearly has more mass than the feather, yet the amount of gravity (force) is the same for each.
The elephant has the greatest acceleration, yet the amount of gravity is the same for each.

13. Think about it!
However, if you drop a book and a piece of paper, you will notice that the paper does not accelerate as much as the book because the small force of gravity on the paper is easily opposed by the force of air friction, fluid friction on the paper. 

14. Galileo
Galileo theorized that in the absence of air, all things would truly fall with the same acceleration.  300 years later the crew of Apollo-15 demonstrated this on the Moon (which has gravity but lacks air) by dropping a hammer and a feather.
Apollo 15 Hammer and Feather

15. Let’s Say it Again So it is Crystal Clear!
All objects free fall at the same rate of acceleration, regardless of their mass!