1. Gravity and Motion What You’ll Do
• Explain how gravity and air resistance affect the acceleration of falling objects.
• Explain why object in orbit appear to be weightless.
• Describe how an orbit is formed.
• Describe projectile motion.

2. Galileo and Falling Objects Video
1500’s - Italian scientist Galileo proved that the mass of an object does not affect the rate at which it falls.
Galileo and Falling Objects Video

4. Acceleration Due to Gravity
Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects.
9.8 m/s/s: This means that for every second that an object is falling, the object’s downward velocity increases by 9.8 meters per second.

5. A falling object accelerates at a constant rate.
Each second the object falls faster and farther than it did the second before.

6. Its downward velocity when it hits the ground is
Math Break
Velocity of Falling Objects
A stone at rest is dropped from a cliff and it takes 3 seconds to hit the ground.
Its downward velocity when it hits the ground is

7. A penny is dropped from the top of a tall stairwell.
Now It’s Your Turn:
A penny is dropped from the top of a tall stairwell.
What is the penny’s velocity after it has fallen for 2 seconds?
The penny hits the ground in 4.5 seconds.
What is its final velocity?

8. Answers
19.6 m/s
44.1 m/s

9. Air Resistance Slows Down Acceleration

11. This fluid friction is also know as air resistance.
The flat paper fell more slowly because of fluid friction that opposes the motion of objects through the air.
This fluid friction is also know as air resistance.

12. Acceleration Stops at Terminal Velocity
The amount of air resistance increases as the speed of the falling object increases.
The upward force of air resistance continues to increase until it exactly matches the downward force of gravity on the object.

13. Terminal Velocity
When this happens the net force is zero and the object stops accelerating.
The object then falls at a constant velocity which is called terminal velocity.

14. This can only happen if there is no air.
Free Fall
Free fall occurs when there is no air resistance.
An object is in free fall only if gravity is pulling it down and no other force is acting on it.
This can only happen if there is no air.

15. Orbiting Objects are in Free Fall
If you traveled through space far away from all the stars and planets you would be so far away from anything the gravitational force on you would be undetectable.
You would still have mass so you would also still have weight - even if just slightly due to some pull of gravity.

16. Two motions combine to cause orbiting -
Astronauts float inside orbiting spacecraft because of free fall not weightlessness.
Two motions combine to cause orbiting -
The spacecraft moves forward at a constant speed
The spacecraft is in free fall because gravity is pulling it down toward Earth

17. The actual path of the spacecraft follows the curve of the Earth’s surface. This is known as orbiting.
Planets, moons, stars and suns orbit larger masses in the center of the galaxy.
All of these objects are traveling in a circular motion - which means they are constantly changing direction

18. Gravity provides the centripetal force that keeps objects in orbit.
An unbalanced force is necessary to keep any object moving in circular motion.
The unbalanced force that causes objects to move in a circular path is called centripetal force.
Gravity provides the centripetal force that keeps objects in orbit.

19. Centripetal Force

20. Projectile Motion
The curved path an object follows when thrown or propelled near the surface of the Earth.
Has two components - horizontal and vertical
When they are combined they form a curved path.

21. Two motions combine to form projectile motion.

22. Horizontal Motion - motion that is parallel to the ground
When you throw a ball your hand exerts a force on the ball that makes it move forward.

23. Vertical Motion - motion that is perpendicular to the ground
• after you throw the ball gravity pulls it down giving it vertical motion.
• because objects in projectile motion accelerate downward, you always have to aim above a target if you want to hit it