1. Free fall occurs whenever an object is acted upon by gravity alone.

2. Object that is dropped…
What do you notice about the motion of the object?
Initial speed is zero
The speed increases as it falls
The longer it falls the faster it travels
The object is accelerating.

3. Object tossed upward…
What do you notice about the motion of the object?
On the way up its speed decreases
At the top, it stops and reverses direction.
The speed increases as it falls back down.

4. Why does this happen?
What causes things to move downward towards the Earth?
Acceleration due to gravity
Gravity pulls objects toward the earth
Free fall occurs whenever an object is acted upon by gravity alone.

5. Why?
What are the factors do you think affect how fast an object will fall to the ground?

6. Let’s test it…
What happens when two objects of different masses dropped at the same time?
The objects will hit the ground at the same time

7. Galileo & the Leaning Tower of Pisa
In the Sixteenth Century, it was assumed that the acceleration of a falling object would be proportional to its mass.
Example: a 10 kg object was expected to accelerate ten times faster than a 1 kg object, according to Aristotle.
Galileo Galilei ( ) came along and put Aristotle's theories to the test.

8. Galileo’s Experiment
Galileo verified his theories through experimentation and careful observation.
Galileo dropped two objects with unequal masses from the Leaning Tower of Pisa.
The two objects struck the ground at the same time.

9. The Acceleration Due to Gravity
gravity on Earth is…
a = 9.81 m/s²
a = 22 mph or 32 feet/s²
The acceleration due to gravity varies with location.
greater at the poles than at the equator.
greater at sea level than in higher elevations.

10. Paper and Pencil Experiment
How are the speed of the flat paper and pencil different?
air resistance causes the paper go slower
What happens if the paper in a ball instead of flat?
They hit the ground at the same time.

11. Terminal Velocity
A free-falling object reaches terminal velocity when the downward force of gravity equals the upward force of air resistance.
120 mph or 55 m/s
Example Skydiving
(videos)