1. Chapter 6
Forces and Motion
Section 1
Gravity and Motion

2. Essential Questions for 6-1
What are the effects of gravity and air resistance on falling objects?
Why are objects in orbit in free fall?
Why do they appear to be weightless?
How is projectile motion affected by gravity?

3. Gravity and Falling Objects
Objects fall to ground at the same rate because acceleration due to gravity is the same for ALL objects
Why? Acceleration depends on both force and mass.
A heavier object experiences a greater gravitational force, BUT it is also harder to accelerate.
Galileo was a genius.

4. Acceleration and Velocity
Acceleration is the rate velocity changes over time. The acceleration of an object affected by gravity is 9.8 m/s.
Change in velocity of falling objects can be measured by the following equation:
∆v = g x t
OR 9.8 m/s times the number of seconds an object falls…

5. Air Resistance
Air resistance – the force that opposes the motion of objects through air
Amount of air resistance depends on the size, shape, and speed of the object

6. Discussion
Why would a feather dropped from the same height as an acorn fall to Earth more slowly than the acorn?

7. Discussion
How can a sky diver benefit from air resistance?

8. Terminal Velocity Terminal Velocity – when air resistance = gravity
0 N or a balanced force!
It’s the fastest an object will fall
It’s also why rain drops don’t kill you… think about it…

9. Free Fall
Free fall - when gravity is the only force acting upon an object
Can only happen where there is NO air resistance:
SPACE (or a vacuum)
- Orbiting objects are in
free fall. Orbit is caused
by two motions:
- Orbiting objects
move forward, but are
also in free fall – so are
the astronauts

10. Discussion
How does free fall in an orbiting spacecraft cause the appearance of weightlessness?

11. Orbiting and Centripetal Force
Orbit is caused by two things that create an unbalanced force
Gravity provides centripetal force to the orbiting object
Centripetal force – the unbalanced force that causes an object to move in a circular path
Ex: planets around the sun or the moon around the Earth

12. Discussion What does free fall in an orbit look like? Which is
The path of the orbiting spacecraft?
The path of free fall for the spacecraft?
The path the spacecraft would follow if there was no gravity?
The curve of the Earth’s surface?

13. Projectile Motion
The curved path an object follows when it is thrown near the surface of the Earth
Composed of horizontal motion and vertical motion
Horizontal motion (like throwing a ball) causes the object to go forward
Vertical motion (gravity) causes the object to fall
Together they create a curved path: BUT still hits the ground the same time as a dropped object
This is why you always have to aim above a target when trying to hit it…

14. An example of projectile motion

15. Quiz 6-1: 4 Questions
1. If a baseball and a cannonball are dropped from the same height at the same time, and there is no air resistance, which ball will hit the ground first? a. The cannonball lands first. b. The baseball lands first. c. The balls land at the same time. d. The ball with the larger volume lands first.

16. 2. What feature of an object does not affect air resistance. a
2. What feature of an object does not affect air resistance? a. its size b. its chemical properties c. its shape d. its speed

17. 3. Why do astronauts in space appear weightless?
There is no gravity in space.
b. Air resistance opposes gravity.
c. They have no mass in space.
d. They are in free fall.

18. 4. In what directions is a projectile accelerated. a
4. In what directions is a projectile accelerated? a. both vertically and horizontally b. vertically downward c. vertically upward d. horizontally forward