1. Learning Objectives
I can relate the force of gravity to the mass of an object.
I can express the relation(s) among mass, distance, and gravitational force.
I can describe the universality of gravity.
I can explain how gravity can keep objects in orbit.

2. Ohio New Learning Standards for Science
Ohio 8th Grade, Physical Science #1 (8.PS.1) – Forces between objects act when the objects are in direct contact or when they are not touching.
Ohio 8th Grade, Physical Science #2 (8.PS.2) – Forces have magnitude and direction.
Ohio 8th Grade, Scientific Inquiry & Application #5 (8.SIA.5) – Develop descriptions, models, explanations & predictions.

3. Lesson Opener 
The number of stories might be different, the need for staircases, placement of shelves, and sturdier foundations would be needed.  
Falling without touching anything.

4. Unit 7, Lesson 2 – Gravity and Motion
What is gravity?
- gravity is a force of attraction between objects due to their mass
- gravity is a noncontact force that acts between two objects at any distance apart
- gravity is an invisible force field that exists everywhere in the Universe!

5. Unit 7, Lesson 2 – Gravity and Motion

6. Unit 7, Lesson 2 – Gravity and Motion
Gravity is a force that can bend and warp space-time!

7. Unit 7, Lesson 2 – Gravity and Motion
- gravity is considered a pulling force, not a pushing force
- you feel the force due to Earth’s gravity as the heaviness of your body, or your weight
- weight is a force and it depends on mass, so greater mass results in greater weight

8. Unit 7, Lesson 2 – Gravity and Motion
- this force of gravity between Earth and an object is equal to the mass of an object m multiplied by a factor due to gravity g
F = mg
- on Earth, gravity’s acceleration is about 9.8 m/s²
- therefore, all objects accelerate towards Earth at the same rate, regardless of mass, if air resistance is removed!
Complete #5, pg.424

9. Unit 7, Lesson 2 – Gravity and Motion
There must be NO air resistance!

10. Unit 7, Lesson 2 – Gravity and Motion
- since all matter has mass and gravity is a result of mass, all matter is affected by gravity!
- everything is attracted to everything else through gravity, but it is not a very powerful force
Why don’t you get gravitationally pulled to the person sitting next to you?
Answer: The gravitational pull of the Earth is greater, so you are being pulled to its core!!!

11. Unit 7, Lesson 2 – Gravity and Motion
- when enough mass gathers together, its effect can be large
- celestial objects that are spherical, like Earth, stars, moons, and galaxies are formed into this shape because of gravity
- some celestial objects do not have enough mass to pull themselves into spheres, though
Deimos, one of Mars’ moons

12. Unit 7, Lesson 2 – Gravity and Motion
- all galaxies move because of the gravitational pull found in the black holes at the heart of the galaxies
Milky Way Galaxy stars revolving
Sombrero Galaxy (M104)
Whirlpool Galaxy (M51a)

13. Whirlpool Galaxy (M51a) coalescing with another galaxy (M51b)
Black Holes the stars revolve around
Whirlpool Galaxy (M51a) coalescing with another galaxy (M51b)

14. Unit 7, Lesson 2 – Gravity and Motion
What determines the force of gravity?
- the Law of Universal Gravitation relates gravitational force, mass, and distance together
- it states that all objects attract each other through gravity, but the strength of the force depends on the masses involved & the distance between them

15. Unit 7, Lesson 2 – Gravity and Motion
- the gravitational force between two objects increases as the distance between their centers decreases
- this is why the Moon is attracted to Earth and not the Sun!

16. Unit 7, Lesson 2 – Gravity and Motion
- the gravitational force between two objects increases with the mass of each object
- this is why your weight would be 1/6 on the moon as it would be on Earth
- a 150 pound person would only weigh 25 pounds on the Moon!
Complete #9 & #10, pg.427

17. Unit 7, Lesson 2 – Gravity and Motion
Jupiter
Earth
Venus
Mars
Mercury also has a smaller radius than Mars has, so the distance used in the weight calculation is smaller, making the attractive force larger.

18. Unit 7, Lesson 2 – Gravity and Motion
How does gravity keep objects in orbit?
- something is in free fall when gravity is pulling it down and no other forces are acting on it
- an object is in orbit when it travels around another object in space
- when shuttles, satellites, or telescopes are orbiting Earth, they are moving forward while being in free fall

19. Unit 7, Lesson 2 – Gravity and Motion
- the combination of the forward motion and downward motion due to gravity combine to cause orbiting
- objects in orbit are traveling incredibly fast, close to 17,000 mph!
- objects won’t stay in orbit forever because of orbital decay, though!
Shuttle Discovery servicing the Hubble Telescope
Complete #11 & #12, pg.428

20. Unit 7, Lesson 2 – Gravity and Motion
The object would fly off into space along a straight line.

21. Unit 7, Lesson 2 – Gravity and Motion
- all objects in space travel in circular or elliptical orbits around other objects
- as an object moves along a curve, it changes direction constantly due to an unbalanced force, which is the force of gravity
The ball travels in an orbit around you. The tension in the string provides the force.
Complete #13, pg.429

22. Unit 7, Lesson 2 – Gravity and Motion

23. Unit 7, Lesson 2 – Gravity and Motion

24. Visual Summary True True False – stronger!
False, the force keeping the moon orbiting the Earth is gravity.
False – stronger!

25. Exit Question
If Earth were replaced by an object with the same mass but much smaller in size, would the moon continue to orbit the new object, fall into it, or fly off into space?
The Moon would continue to orbit the new object!