1. Gravitational Fields and Interactions
Or I’m very attracted to you…
Standards:
1e Students know the relationship between the universal law of gravitation and the effect of gravity on an object on the surface of Earth.

2. Gravitational Fields
Gravity is an action at a distance force meaning it acts on things even if there is no touching.
A mass changes the space around it so that other objects will feel a gravitational force. This change is called a gravitational force field.
Fg/ mtest =g=gravitational force field variable = acceleration of gravity = G msource /d2

3. Weight Weight is the force of gravity on an object
F = G M earth x m object / radius earth 2
Gravitational field = g=GMe/Re2
W=mg near earth’s surface
For other planets
F and g have a direct and linear relationship with mass
F and g have an inverse square relationship with the radius of the planet.
Question: Define weight. Write the mathematical equation for the gravitational field variable. How is the gravitational field of a planet affected by the mass and radius of the planet?

4. Check Question
Two planets have the same mass. The radius of planet A is 4 times the radius of planet B. How much does an object that weighs 10 N on planet A weigh on planet B? A B

5. Tides
Tides are caused by difference in gravitational pull on different parts of Earth
Greater difference results in greater tides
Both Sun and Moon create tides but Moon’s are greater even though Sun’s force is greater
Spring and Neap tides (see next slide)
Questions: What causes the tides on Earth ? Which causes larger tides the Sun or Moon? How would Earth’s tides be affected by the following; a) increasing the distance between the Earth and Moon? b) increasing the mass of the Moon? C) Increasing the diameter of the Earth?

6. Neap and Spring Tides
Question: Draw the location of the Sun, Moon and Earth during a neap and spring tide? Which produces the highest high tides and lowest low tides?

7. Check Question
How would doubling the following quantities affect the tides on the Earth?
The mass of the Moon
The diameter of the Moon
The distance between the Moon and Earth
The diameter of the Earth
What two phases of the Moon are possible during a spring tide?

8. Apparent Weightlessness
Not due to being too far from planet.
Occurs when there is an absence of support force or an object is in free fall
Gravity actually is only force acting
Examples
Spacecraft in orbit
Falling out of an airplane or stepping off step
Car or plane in Whoop de doos
Elevator of Death
Questions: Why do astronauts appear weightless when traveling in an orbiting space craft? What are some other examples of this phenomenon?
Activity:
1) Show Video: Zero g
2) Do elevator of doom example
3) Discuss Vomit Comet and how you can get same effect by driving car over whoop de doos.
4) Discuss carnival rides free fall and what a dropped penny would do.
5) Finish by stating astronauts appear weightless because they are in free fall such that all the objects inside the cabin and the cabin itself are falling at the same rate. Thus there is no support force between these objects.

9. Black Holes Form from large used up stars that collapse
Curvature of space and time
Tidal forces
Time dilation
Seen by Hubble in 1996
Also formed after big bang and were seeds for galaxies
What do black holes form from? Have they been observed? What are tidal forces?

10. Check Question
What would happen to the Earth’s orbit if the Sun suddenly became a black hole?
It would decrease in diameter and be pulled into the black hole
It would increase in diameter eventually escaping the black hole’s gravitational field
It would remain in its same orbit with the same speed.

11. Gravitational Field of Earth
Outside of Earth
Follows inverse square law
Decreases as move outward
Never zero
Inside Earth
Zero at center
greatest at surface of planet where g=9.81 m/s/s
Tunnels and destroying the Earth w/ class
Question: Draw a graph showing the gravitational field inside and outside the Earth. Where would you have the greatest weight? How about the least?

12. Gravitational Fields
Mass changes space around it so other masses feel a force due to gravity (gravitational field)
Measured using gravitational field variable
Force / test mass = g in N/kg
Represented by force lines
Varies w/ source mass and distance but not test mass
Any mass creates a gravitational field around it.
Questions: How does a mass change the space around it? What variable and unit is used to measure a gravitational field? What variables does the magnitude of a gravitational field depend upon?