1. Gravity and Motion

2. Aristotle (400B.C.)
Thought that an objects mass determined how fast it would fall.
If you dropped a baseball and a marble at the same time, he would have predicted that the baseball would land first.

3. Galileo (Late 1500’s)
Young Italian Scientist who questioned Aristotle’s idea about falling objects.
Proved that the mass of an object does NOT affect the rate at which it falls.
Proved this by dropping two cannonballs of different masses from the top of the Leaning Tower of Pisa.
The crowd witnessed the two cannonballs landing at the SAME time

4. Galileo’s idea changed people’s understanding of gravity and falling objects.

5. All Objects Fall with the SAME Acceleration!
Objects fall to the ground at the same rate because the acceleration due to gravity is the same for ALL objects.
The force of gravity is the same between Earth and an object with a large mass than between Earth and a less massive object.
You may think that the acceleration due to gravity should be greater too, BUT a greater force must be applied to a large mass than to a small mass to produce the same acceleration.
The difference in force is canceled by the difference in mass.

6. Accelerating at a Constant Rate
All objects accelerate toward Earth at a rate of 9.8 m/s2
This means that for every second that an object falls, the objects downward velocity increases by 9.8 m/s.
This acceleration is the same for all objects regardless of their mass.

7. Velocity of Falling Objects
To find the change in velocity (∆v) of a falling object, multiply the acceleration due to gravity (g) by the time it takes for the object to fall in seconds (t):
FORMULA:
∆v = g x t

8. Try this one!
A stone at rest is dropped from a cliff, and it takes 3 seconds to hit the ground. What is its downward velocity when it hits the ground?
Remember, ∆v = g x t
∆v = 9.8 m/s/s x 3 s
∆v = 29.4 m/s

9. And some more!!
What is a penny’s velocity after it has fallen for 2 seconds?
A marble at rest is dropped from a tall building. The marble hits the ground with a velocity of 98 m/s. How long was the marble in the air?
An acorn at rest falls from an oak tree. The acorn hits the ground with a velocity of 14.7 m/s. How long did it take the acorn to land?

10. Air Resistance Slows Down Acceleration
Drop 2 pieces of paper - one crumpled in a tight ball and the other kept flat.
What kept the flat paper from falling at the same rate as the crumpled paper?
Fluid friction is also known as air resistance.
Air resistance occurs between the surface of the falling object and the air that surrounds it.

11. Air Resistance
The amount of air resistance acting on an object depends on the size and shape of the object.
Air resistance affects the flat sheet of paper more slowly than the crumpled one, causing the flat sheet to fall more slowly than the crumpled one.
Any falling object you see is affected by air resistance.

12. Represents the force of air resistance pushing up on the object
Represents the force of air resistance pushing up on the object. This force is subtracted from the force of gravity to produce the net force.
Represents the net force on the object. Because the net force is not zero, the object still accelerates downward, just not as fast as it would without air resistance.
Represents the force of gravity on the object. If there wasn’t air resistance, the object would accelerate at a rate of 9.8 m/s/s.

13. As long as the net force on a falling object is not zero, the object accelerates downward.
The amount of air resistance on an object increases as the speed of the object increases.
As an object falls, the upward force of air resistance continues to increase until it exactly matches the downward force of gravity.

14. Terminal Velocity
When the air resistance (upward force) matches the downward force of gravity, the net force is zero, and the object stops accelerating.
The object then falls at a constant velocity, which is called the terminal velocity.

15. Free Fall
Sky divers are often described as being in free fall before they open their parachutes.
This in an incorrect description, because air resistance is always acting on the sky diver.
An object is in free fall only if gravity is pulling it down and no other forces are acting on it.

16. Free Fall Cont’d
Because air resistance is a force (fluid friction), free fall can occur only where there is no air - in a vacuum (a place in which there is no matter) or in space.
Astronauts “float” in orbiting spaceships because of free fall.

17. Orbiting
An object is said to be orbiting when it is traveling in a circular or nearly circular path around another object.
The moon orbits the Earth, Earth and other planets orbit the sun, and many stars orbit large masses in the center of the galaxies.
Any object in a circular motion is constantly changing direction.

18. Because an unbalanced force is necessary to change the motion of any object, there must be an unbalanced force working on any object in a circular motion.
The unbalanced force that causes objects to move in a circular path is called centripetal force.

19. Centripetal Force
Gravity provides the centripetal force that keeps objects in orbit.
The word centripetal means “toward the center”.

20. Types of Motion due to Gravity
Projectile Motion - the curved path an object follows when thrown or propelled near the surface of the Earth.
Has 2 components- horizontal and vertical. When the 2 motions are combined, they form a curved path.

21. Horizontal Motion - motion that is parallel to the ground (side to side).
Vertical Motion - motion that is perpendicular to the ground (up and down).

22. The downward acceleration of a dropped object versus a thrown object are the same (if air resistance is ignored).
They both are being pulled down by gravity with the acceleration of 9.8 m/s2.

23. Answer on a Separate Sheet of Paper!
How does air resistance affect the acceleration of falling objects?
How does gravity affect the 2 components of projectile motion?
How is the acceleration of falling objects affected by gravity?
A rock at rest falls off a tall cliff and hits the valley below after 3.5 s. What is the rock’s velocity as it hits the ground?