1. Learning Target 5 → Free Fall: I can use the acceleration of gravity to describe and calculate an object's motion
5.1 I can describe how and why the rate of all objects falling to earth is 9.81 m/s2
5.2 I can calculate the speed or distance an object fell for any length of time
5.3 I can calculate the maximum height of an object thrown in the air and the speed and time for its return path

2. As a Human: This is true college prep material!
Free Fall
Why This Matters
As a Human: This is true college prep material!
As a Scientist:
You will be applying your knowledge of kinematic equations to a new topic – this is thinking like a scientist.

3. Free fall
Definition: downward movement under the force of gravity only

4. Gravity
Definition: the force that attracts a body toward the center of the earth, or toward any other physical body having mass
When you drop an object, gravity starts pulling it down
Acceleration due to gravity is always down toward the center of the earth
Acceleration due to gravity represented as “g” and is equal to 9.8 m/s2

5. The acceleration is the same at all points – only the velocity changes.
Even though the velocity is zero at the top of the ball’s flight, its acceleration is still -9.8 m/s2.
If you know the initial velocity at which the ball was thrown, you can calculate how fast it is going and how far it goes at any point during its flight.

6. Velocity vs time graph Velocity- m/s
-downward slope indicates acceleration is constant at every moment.

7. Sum it up Position/Sign Acceleration/Sign Velocity/Sign
On the way up (+)
9.8 m/s2 (-)
Decreasing (-)
At the top (o)
0 (o)
On the way down (-)
Increasing (+)

8. The wonders of free fall
In the absence of air resistance, all objects fall at a constant acceleration, regardless of their size or weight.
Paper/Book Demo
Why does the book hit the ground before the paper?

9. Don’t Believe Me? http://www.youtube.com/watch?v=KDp1tiUsZw8

10. Quick Checks
Q: What is the only force acting on an object in free fall? A: Gravity Q: What is the acceleration due to gravity? A: -9.8 m/s2 Q: Why is the acceleration due to gravity negative? A: Because it’s directed downward Q: Why does a piece of paper fall to the ground after a text book? A: Air resistance – in a vacuum they would fall at the same time!

11. Quick Checks
When an object is thrown up into the air… Q: What happens to the acceleration on its way up? A: Stays the same (= -9.8 m/s2) Q: What happens to the velocity on its way up? A: Decreases Q: What is the velocity at the top of the flight? A: 0 m/s Q: What is the acceleration at the top of the flight? A: -9.8 m/s2 Q: What happens to acceleration on the way down? Q: What happens to the velocity on its way down? A: Increases

12. vf = vi + at
So let’s say the ball was thrown with an initial velocity of 10.5 m/s. How fast was it going after 1 s?

13. Kinematic Equations d = vit + ½ at2 vf = vi + at d = (vi + vf) 2 t
Displacement = d
Initial velocity = vi or vo
Final velocity = vf
Time = t
Acceleration = a
vf = vi + at
d = (vi + vf) 2 t
vf2 = vi2 + 2ad

14. Δy = vit + ½ at2
While watching the HHS football team play at Farrington Field, I accidentally dropped my cell phone from my seat at the top of the bleachers. I heard it crack 3.3 seconds later. How high up was my seat?

15. Δ y= vit + ½ at2
A ball is thrown vertically upward with an initial velocity of 19.5 m/s. What is the maximum height reached by the ball 4 seconds later?

16. Δy = vit + ½ at2
A 30 lb weight falls freely from rest from the roof of a building. What is the total distance the weight falls after 3 seconds?

17. Closing Question
Knowing what you know about free fall, predict what the position vs. time graph and velocity vs. time graph would look like for an object in free fall?