Transcript Section 7.1 - CPO Science

Laws of Motion and Energy
Chapter Seven: Gravity and
Space
• 7.1 Gravity
• 7.2 The Solar System
• 7.3 The Sun and the Stars
7.1 Gravity
• Gravity creates an attractive force
between all objects that have mass.
7.1 Gravity
• There is even a
gravitational force
between you and a
book.
• You don’t notice gravity
between ordinary
objects because it
takes a huge amount of
mass to create a force
big enough to feel.
7.1 Conserving Energy
• The gravitational force
between you and Earth
is much stronger than
between you and your
book.
7.1 Law of gravitation
• Newton’s law of universal gravitation says the
strength of the force depends on mass and
distance.
• If two objects are the same mass they have the
pull on each other with the same amount of force
(Fg).
7.1 Law of gravitation
• If one object doubles in mass, then the
gravitational force doubles.
7.1 Law of gravitation
• If both objects double in mass, then the
force doubles twice, becoming four times
as strong.
7.1 Weightlessness
•
•
•
The astronauts in the space shuttle are not far
enough away to escape Earth’s gravity.
How do they float around with no apparent
weight?
The answer is that they are actually falling
around the Earth in their orbit.
7.1 Weightlessness
• An elevator
accelerating upward
makes you heavier.
• A scale in the elevator
would show your weight
plus the additional force
applied to accelerate
you.
7.1 Weightlessness
• The scale “falls away”
beneath your feet with the
same acceleration as you
fall.
• The scale would show zero
because there would be no
force between it and your
feet.
7.1 Orbital motion
• Newton’s second law
explains the motion of
planets, moons, and
satellites.
• An orbit is the circular (or
elliptical) path an object
takes around a gravity
source, such as a planet or
star.
7.1 The Earth and Sun
• Compared to Earth,
the Sun is huge.
• The Sun has
330,000 times the
mass of Earth and
is 110 times larger
in diameter.
7.1 The Earth and Sun
• At a distance of 150
million km, an 80-kg
person is attracted to the
Sun only with a force of
about half a newton.
• Earth’s gravity gives the
same person a weight of
784 newtons.
7.1 The Earth and Moon
• The Moon is much
smaller than Earth.
• Because the Moon
has much less
mass, its surface
gravity is about 1/6
as strong as the
gravity on Earth.
7.1 The Earth and Moon
• The orbit of the Moon
is tilted about 5o from
the orbit of Earth.
• This means the Moon
is not in Earth’s
shadow except for
rare eclipses.
7.1 The Earth and Moon
• The Moon’s own
shadow creates the
phases that we
see.
7.1 Tides
• A daily rise in ocean level is called a tide.
• Tides are caused by the gravitational pull of the
Moon acting on the water in the oceans.
• The moon-side tide is caused by the moon’s
gravity pulling on the ocean.
7.1 Tides
• As the Earth rotates around the center of
mass, the ocean on the opposite side from
the Moon is “flung outward” a little by its
own inertia.
How many tides
are there every
24 hours?