1. Chapter 2 Pages 36-59 http://www.physicsclassroom.com/mmedia/
Forces and Motion
Chapter 2
Pages 36-59

2. Gravity and Motion
Greece 400 BC, Aristotle thought the rate at which an object would fall was based on its mass. He never tested his hypothesis.

3. Gravity and Falling Objects
Late 1500s, Galileo quested Aristotle's idea.
Galileo tested two cannonballs with different masses drop from the Leaning Tower of Pisa. Supposedly , people watching claimed both cannonballs landed at the same time.
Galileo’s work changed how people understood how gravity of falling objects.
Iron ball
Wood ball

4. Gravity and Acceleration
Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects.
WHY???
Acceleration depends on force and mass.
YES, a heavier object experiences a greater gravitational force BUT it also is harder to accelerate SO it balances out.

5. Acceleration due to Gravity
Acceleration is the rate at which velocity changes over time.
Falling objects accelerate at a constant rate.
Objects falling to Earth accelerate at a rate of 9.8 meters per second, or 9.8 m/s/s or 9.8 m/s²

6. Velocity of Falling Objects
Calculate the change in velocity (Δv)
The change in velocity is the difference between the final velocity and the starting velocity.
Formula:
Δv=g x t
g is the acceleration due to gravity on Earth (9.8 m/s²)
t is the time the object falls in seconds

7. Examples of Velocity of Falling Objects Page 37 examples 1-4
Example 1: Δv= 9.8 m/s² x 2 s
Answer is 19.6 m/s downward
Example 2: Δv= 9.8 m/s² x 4.5 s
Answer is 44.1 m/s downward
Example 3: 98 m/s ÷ 9.8 m/s² = t
Answer is 10 s
Example 4: 14.7 m/s ÷ 9.8 m/s² = t
Answer is 1.5 s
(please note that 1 m/s is approx. 2 miles per hour)

8. Air Resistance and Falling Objects
Air resistance is the force that opposes the motion of objects through air. (this is why a crumpled piece of paper falls faster than a flat sheet of paper because of surface area)
Air resistance depends on SIZE, SHAPE, and SPEED of an object.

9. Acceleration Stops at the Terminal Velocity
Terminal velocity is the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity.
Terminal velocity is good for slowing down hail and sky diving.

10. Free Fall Occurs when there is NO Air Resistance
Free Fall can only when there is NO air. Either space or a vacuum (a place with no matter).
Free Fall is the motion of a body when only the force of gravity is acting on the body

11. Orbiting Objects are in Free Fall
Astronauts float in orbiting spacecrafts because of free fall NOT because they are weightless.

12. Two Motions Combine to Cause Orbiting
An object is orbiting when it is traveling around another object in space.
See example on page 40.
Spacecrafts and satellites
The Moon orbits Earth
Earth orbits the Sun

13. Orbiting and Centripetal Force
Any object in a circular motion is constantly changing direction. Because an unbalanced force is necessary to change the motion of any object, there must be unbalanced force working on any object in circular motion.
The unbalanced force that causes the objects to move in a circular path is known as centripetal force. The word centripetal means “toward the center.”
This is why the Moon stays in orbit around Earth.

14. Projectile Motion and Gravity
Projectile motion is the curved path an object follows when it is thrown or propelled near the surface of the Earth. It has two components- horizontal and vertical motion, both independent of each other.
Examples: frog leaping, water sprayed by a sprinkler, swimmer diving into water, balls being juggled, an arrow shot by an archer

15. Horizontal Motion
Throwing a ball your hand exerts a force on the ball that makes the ball move forward. This gives the ball its horizontal motion, which is parallel to the ground.
The ball’s horizontal velocity remains constant but the ball’s vertical velocity increases because of gravity causing it to accelerate downward …these two motions cause the curved path.

16. Vertical Motion
Vertical motion is motion that is perpendicular to the ground.
Gravity gives vertical motion to an object in projectile motion.

17. Newton’s Laws of Motion
1st Law of Motion: An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line UNLESS acted on by an unbalanced force.

18. Newton’s 1st Law (a.k.a. the Law of Inertia)
Objects will remain at rest, not moving UNLESS…
Objects will continue to move at specific velocity UNLESS…
Friction is usually the unbalanced force that causes objects to stop
Law of Inertia: tendency for all objects to resist ANY change (either starting or stopping) motion

19. Interactive
Play 1st law

20. NEWTON’S 1st LAW OF MOTION
EXAMPLE # 1
What causes the motion of the car to change?

21. NEWTON’S 1st LAW OF MOTION
EXAMPLE # 2 – Explain why the test tube breaks.

23. Newton’s 2nd Law of Motion
The acceleration of an object depends on the mass of the object and the amount of force applied.
As mass increases/ acceleration decreases and vice versa
Acceleration increases as the force increases
F = m x a

24. Newton’s 2nd Law Try Math Focus on page 48 a = F ÷ m
68.8 N ÷ 7 kg = 9.8 m/s²
2. F = m x a
1250 kg x 40 m/s² = N
3. F = m x a
175 kg x 2 m/s² = 350 N

25. Second Law Look at the animation. Why do the elephant and
feather hit the ground at
the same time?
(assume no air resistance)
TEKS: 8.3(b), 8.7(a)
Source:

26. Newton’s 3rd Law of Motion
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first
Force pairs do not act on the same object
All forces act in pairs – action and reaction
The effect of a reaction can be difficult to see

27. Third Law
Look at the animation. This shows an action with an equal but opposite reaction.
Can you think of any other equal but opposite force pairs?
TEKS: 8.3(b), 8.7(a)
Source:

28. Momentum
The momentum of an object depends on the object’s mass and velocity
p = m x v
p is momentum, m is mass, v is velocity
Try the # 1 on Math Focus pg 53:
p = 6 kg x 10 m/s
p = 60 kg·m/s

29. Law of Conservation of Momentum
That any time objects collide, the total amount of momentum stays the same.
This law applies whether the objects stick together or bounce off each other after they collide
Objects can either stick together OR bounce off each other after a collision
Conservation of Momentum can be explained with Newton’s 3rd Law- think billiard balls