1. “Universal Gravitation”
Gravity
“Universal Gravitation”

2. Presentation Outline Learning Target: Proportional Reasoning
Newton and the Apple  Gravity is universal
Invisible long range force that goes out in all directions – similar to light
Intensity of Light drops off by the square of the distance (inverse square)
Gravity has the same inverse square relationship with distance (Gravity Trailer)
What else determines the strength of gravity? (Mass)
Gravity is proportional to the product of the masses and inversely proportional to the square of the distance.
Examples and practice

3. How do we make sense of the natural world?
Central Question
How do we make sense of the natural world?

4. How do we make sense of the natural world?
Central Question
How do we make sense of the natural world?
GRAVITY

5. Learning Targets
5.1 – I can use proportional reasoning to predict changes in gravitational forces based on changes in masses and/or separation distance.

6. Newton and the Apple
According to legend, Isaac Newton sat under an apple tree and looked up into the sky—and this led him to discover that gravity was universal.
In Newton’s time, the laws of nature on the Earth were not the same as the laws of nature in the sky.
Newton was the first to reason that if the Earth pulls an apple down, it also pulls the Moon around the Earth—this would explain how the Moon constantly accelerates as it travels in a circle!
Newton also reasoned that the Moon must pull back on the Earth, which would explain why tides matched lunar cycles.

7. Newton and the Apple
According to legend, Isaac Newton sat under an apple tree and looked up into the sky—and this led him to discover that gravity was universal.
In Newton’s time, the laws of nature on the Earth were not the same as the laws of nature in the sky.
Newton was the first to reason that if the Earth pulls an apple down, it also pulls the Moon around the Earth—this would explain how the Moon constantly accelerates as it travels in a circle!
Newton also reasoned that the Moon must pull back on the Earth, which would explain why tides matched lunar cycles.
Newton’s reflections led him to discover that the force of gravity is universal—meaning that gravity exists between any two objects in the universe.

8. So, gravity is a universal force that attracts any two objects together. This force is invisible and must be able to extend in any direction from one mass to another.

9. So, gravity is a universal force that attracts any two objects together. This force is invisible and must be able to extend in any direction from one mass to another.

10. So, gravity is a universal force that attracts any two objects together. This force is invisible and must be able to extend in any direction from one mass to another.
This long-range force weakens with the separation distance between the objects…

11. This is just like the intensity of light!
So, gravity is a universal force that attracts any two objects together. This force is invisible and must be able to extend in any direction from one mass to another.
This long-range force weakens with the separation distance between the objects…
This is just like the intensity of light!

12. The Inverse Square Law
The intensity of light obeys the inverse square law, which says that as the distance from the source increases by a certain factor, the intensity of the light decreases by the square of the factor.

13. The Inverse Square Law
Gravity obeys the same inverse square law—as the distance increases by a certain factor, the strength of gravity decreases by the square of the factor.

14. Does gravity still work in long distances?

15. Factors that influence the strength of gravity
The distance between two objects affects the strength of gravity between them.
What else might affect the strength of gravity between two objects?

16. Factors that influence the strength of gravity
The distance between two objects affects the strength of gravity between them.
What else might affect the strength of gravity between two objects?
The larger the mass of the two objects, the stronger the gravitational force between them!

17. Gravity is proportional to…
The mass of each object
The inverse of the square of the distance (inverse square law)

18. MASS STRENGTHENS GRAVITY
If the mass of one object doubles, the strength of gravity doubles.
If the mass of one object is cut in half, the strength of gravity is cut in half.
If the mass of both objects doubles, the strength of gravity increases four times as much.

19. MASS STRENGTHENS GRAVITY
If the mass of one object doubles, the strength of gravity doubles.
If the mass of one object is cut in half, the strength of gravity is cut in half.
If the mass of both objects doubles, the strength of gravity increases four times as much.

20. MASS STRENGTHENS GRAVITY
If the mass of one object doubles, the strength of gravity doubles.
If the mass of one object is cut in half, the strength of gravity is cut in half.
If the mass of both objects doubles, the strength of gravity increases four times as much.

21. MASS STRENGTHENS GRAVITY
If the mass of one object doubles, the strength of gravity doubles.
If the mass of one object is cut in half, the strength of gravity is cut in half.
If the mass of both objects doubles, the strength of gravity increases four times as much.

22. MASS STRENGTHENS GRAVITY
Example: The mass of one asteroid triples. By what factor does the force of gravity change?

23. DISTANCE WEAKENS GRAVITY
If the distance between the objects doubles, the strength of gravity is one-fourth as strong.
If the distance between the objects is cut in half, the strength of gravity is four times as strong.

24. DISTANCE WEAKENS GRAVITY
If the distance between the objects doubles, the strength of gravity is one-fourth as strong.
If the distance between the objects is cut in half, the strength of gravity is four times as strong.

25. DISTANCE WEAKENS GRAVITY
If the distance between the objects doubles, the strength of gravity is one-fourth as strong.
If the distance between the objects is cut in half, the strength of gravity is four times as strong.

26. DISTANCE WEAKENS GRAVITY
Example: The distance between two asteroids triples. By what factor does the force of gravity change?

27. Ratio Example
Often, we will use ratios to determine by what factor the mass or distance between objects changes. Then, use that information to determine how much the gravitational force changes. For example, let’s say an two asteroids start out 5 meters apart and then are moved to a distance of 35 meters apart. By what factor does the gravitational force between the asteroids change?

28. Ratio Example New Distance 35 meters Old Distance 5 meters
Often, we will use ratios to determine by what factor the mass or distance between objects changes. Then, use that information to determine how much the gravitational force changes. For example, let’s say an two asteroids start out 5 meters apart and then are moved to a distance of 35 meters apart. By what factor does the gravitational force between the asteroids change?
New Distance meters
Old Distance meters
________________ = ____________ =

29. Ratio Example
Often, we will use ratios to determine by what factor the mass or distance between objects changes. Then, use that information to determine how much the gravitational force changes. This time, the asteroids are staying the same distance apart. Both of the asteroids have a mass of 75 kg. One of the asteroid’s mass suddenly decreases to 25 kg. By what factor would the gravitational force on the asteroids change?

30. Ratio Example New Mass 25 kg 1 Old Mass 75 kg 3
Often, we will use ratios to determine by what factor the mass or distance between objects changes. Then, use that information to determine how much the gravitational force changes. This time, the asteroids are staying the same distance apart. Both of the asteroids have a mass of 75 kg. One of the asteroid’s mass suddenly decreases to 25 kg. By what factor would the gravitational force on the asteroids change?
New Mass kg
Old Mass kg
____________ = ______ = ___

31. Gravitational Proportions Practice