1. Free Fall

2. Aristotle Heavy objects fall faster.
Aristotle had a theory that objects fell to earth according to their “nature”. Heavy objects were of a “earth nature” and therefore would fall to the earth faster, since the earth was their natural state. Lighter objects were of an “air nature” so they would remain in the air longer. Aristotle’s views were held as true for thousands of years.
Heavy objects fall faster.

3. Galileo’s Thought Experiment
Galileo conducted a “thought experiment” about falling bodies. It was a thought experiment because it relied on logic rather than actual experimentation. He considered three identical objects. They would fall at the same rate because they are exactly the same.

4. Galileo’s Thought Experiment, Part 2
Galileo then considered the same three objects, but with two joined together by a weightless chain. Nothing had changed about the objects so the should still fall at the same rate. But there were now only two objects– one twice as heavy as the other. This indicated to Galileo that weight did not determine how fast an object fell.

5. Galileo’s Conclusion
All objects will fall at the same rate if air resistance is removed or ignored.
Since weight did not determine how fast an object fell, Galileo determined that it was another factor that determined the rate. This factor is air resistance. Air resists some objects more than other objects which determines how fast the objects fall. If air resistance is removed all objects will fall at the same rate.

6. g The rate at which objects fall is called g. It is approximately
9.8 m/s2
on the surface of the Earth.

7. Equations for Free Fall from Rest
These equations can be used for falling bodies from rest. Notice they are the three equations for constant acceleration linear motion with the initial speed set to zero and the acceleration equal to –g.

8. Free Fall with initial velocity
Speed at highest point is 0
Speed back at initial height = -vi
Time up = time down

9. Problem #1
t = ?
20 m

10. Problem #2
vi=15 m/s
t=?