1. Forces and Motion
Chapter 6

2. Gravity and motion
Section 6:1

3. Gravity and Falling Objects
Background
400 BCE, Aristotle thought that the rate an object falls depends on its mass. He never tested this idea
1500’s Galileo Galilee questioned Aristotle's idea and argued that the mass doesn’t matter
Galileo changed the way people thought

4. Gravity and Acceleration
Objects fall at the same rate because acceleration due to gravity is the sam for all objects
Acceleration depends on force and mass
Heavier objects are harder to accelerate because they have more mass
Extra mass balances out the gravitional forces

5. Acceleration due to gravity
Acceleration is the rate at which velocity changes over time
All objects on Earth accelerate toward Earth at 9.8m/s2
Every second an object falls, its downward velocity increases 9.8 m/s

6. Velocity of falling objects
Change of velocity of a falling objects
V = G x t
Velocity = gravity x time
G = 9.8m/s2
Change in velocity = final velocity – initial velocity
Example
A penny is dropped for the top of a tall stairway. What is the penny’s velocity after 2 seconds? After 4.5 seconds

7. Air Resistance and Falling Objects
Force that opposes motion though air
Depends on the object’s size, shape and speed
Examples
Flat vs. crumpled piece of paper

8. Acceleration Stops at Terminal Velocity
Terminal velocity is the constant velocity of a falling object when the forces of the air resistance is equal in magnitude and opposite in direction as gravity
As an object falls, speed and air resistance increase
Upward air resistance increases until it is equal to the downward pull of gravity
This equals a net force of 0 and the object stops accelerating
Terminal velocity is a good thing
Hailstones normally travel at a velocity between 5-40 m/s
Without air resistance = 350 m/s

10. Free Fall Free fall occurs when there is no air resistance
Only gravity is working on the objects
Only place with no outside force ( air resistance) is space or a vaccuum

11. Orbiting objects are in free fall
Astronauts float because they are in free fall
Gravitational force is very small because the distance is very large
They still have mass
Gravity will attract, if slightly, so technically they are NOT weightless

12. Orbiting When one object is traveling around another object in space
2 motions are present
Shuttle moves forward and it is in free fall
Makes a curved path
Shuttle is falling while in orbit

13. Centripetal Force
Moon or other planets in orbit travel in circular path
Any object in circular path in constantly changing directions
Unbalanced forces need to change directions therefore the unbalanced force is in circular motion
Centripetal force is an unbalanced force that causes objects to move in a circular path

14. Centripetal means “toward center”
Gravity is the centripetal force that keeps planets in orbit

15. Projectile Motion and Gravity
Curved path that an object follows when thrown, launched or otherwise projected near the Earth’s surface
Examples frog leaping, water sprayed by sprinklers, arrow shot by archer
Projectile motion has 2 componets
Horizontal motion
Vertical motion

16. Horizontal Motion Force to put an object in forward motion
baseball
Gravity doesn’t affect horizontal motion
constant

17. Vertical motion Gravity pulls objects down 9.8m/s2
Gravity gives vertical motion
Aim is caused by downward acceleration