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If g is the gravitational field strength at some point (in N/kg), then the free fall acceleration at that point is also g (in m/s2).

Slide 31

If you are located a distance r from the center of a planet:

all of the planet’s mass inside a sphere of radius r pulls you toward the center of the planet.

All of the planet’s mass outside a sphere of radius r exerts no net gravitational force on you.

Slide 32

Gravitational Field Inside a Planet

The blue-shaded part of the planet pulls you toward point C.

The grey-shaded part of the planet does not pull you at all.

Slide 33

Gravitational Field Inside a Planet

Half way to the center of the planet, g has one-half of its surface value.

At the center of the planet, g = 0 N/kg.

Slide 34

When a very massive star gets old and runs out of fusionable material, gravitational forces may cause it to collapse to a mathematical point - a singularity. All normal matter is crushed out of existence. This is a black hole.

Slide 35

Black Hole Gravitational Force

Slide 36

Black Hole Gravitational Force

The black hole’s gravity is the same as the original star’s at distances greater than the star’s original radius.

Black hole’s don’t magically “suck things in.”

The black hole’s gravity is intense because you can get really, really close to it!

Slide 37

There are 2 high tides and 2 low tides per day.

The tides follow the Moon.

Slide 38

Why Two Tides?

Tides are caused by the stretching of a planet.

Stretching is caused by a difference in forces on the two sides of an object.

Since gravitational force depends on distance, there is more gravitational force on the side of Earth closest to the Moon and less gravitational force on the side of Earth farther from the Moon.

Slide 39

Why Two Tides?

Remember that

Slide 40

Why the Moon?

The Sun’s gravitational pull on Earth is much larger than the Moon’s gravitational pull on Earth. So why do the tides follow the Moon and not the Sun?

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