Chapter 7
Gravity
Presented by April Senger
How Does It Happen?
 There is an old tale about Newton sitting under
an apple tree and watching an apple fall
 Newton also realized that the apple and even
the moon were affected by the Earth’s gravity
 So why does an apple fall straight down and
the moon appears to stay in a set position?
Presented by April Senger
What Makes Gravity
Strong?
 Gravity is dependent on two
factors…distance between the objects
and mass
 Gravity is directly proportional to mass
 The distance is inversely proportional to
gravity
Presented by April Senger
Concept Check
 What happens to the force acting between two bodies
if the mass of one body is doubled?
 The force doubles
 What happens if the mass of the other body is
doubled?
 Same
 What happens if the masses of both the bodies are
doubled?
 The forces is 4 x as much
 What happens if the mass of one body is doubled and
the other is tripled?
 It would increase by 6 x
Presented by April Senger
Details on Distance
 Not only are gravity & distance inversely proportional,
they follow the Inverse-Square Law
 This means that if I double the distance, the gravity
decreases by ¼
 If I triple the distance, the gravity decreases by 1/9
 What happens if you quadruple the distance?
 1/16 the force of gravity
 A spray can and its paint, light from a match, radiation
from a piece of uranium, and sound from a cricket all
follow this same law
Presented by April Senger
Centering Our Thoughts
 Remember that gravity is the force that pulls
mass towards the CENTER of the Earth
 Most people assume that if you throw a ball up,
it comes down because the Earth’s surface
pulls it there
 Try digging a hole and see what happens…The
pull is actually towards the center of the Earth
Presented by April Senger
Concept Check
 How much does the force of gravity change between
the Earth and a receding rocket when the distance
doubles? Triples? Ten times as much?
 ¼, 1/9, 1/100
 Consider an apple at the top of a tree. The apple is
pulled by the Earth’s gravity with a force of 1 N. If the
tree were twice as tall, would the force of gravity by
only ¼ as strong?
 Don’t forget the pull is towards the center of the Earth.
Doubling a tree doesn’t compare to the 6,370 km
radius of our planet. The apple would weight nearly the
same
Presented by April Senger
No Escaping It
 As long as two objects have mass, they
will always have gravity
 Think of your Lucky Charms floating in
the milk
 Given time, they will clump and cling to
the sides
 Even two dust particles have gravity
Presented by April Senger
Concept Check
 Light from the sun obeys the inverse-square
law. If you were on a planet twice as far from
the sun, how bright would the sun look?
 ¼ as bright
 How bright would the sun look if you were on a
planet twice as close to the sun?
 4 x as bright
Presented by April Senger
Ocean Tides
 Why does the Earth have tides?
 In order to have the amount of water in the ocean
move that much, there must be a large mass or
masses near by
 The moon rotates to the same position every 24 hrs
and 50 minutes so tides are never exactly the same
time each day
 The moon is much less massive than the Sun
 The sun does pull on our oceans but doesn’t have the
impact that the moon has…Why?
 The differences in pull on the ocean are what cause
the tides
Presented by April Senger
Concept Check
 If the moon didn’t exist, would the Earth still have
tides? If so any idea how often?
 Yes there would be due to the sun. They would occur
twice a day every 12 hours
 We know that both the moon and the sun produce our
ocean tides. The moon plays a greater role because it
is closer. Does its closeness mean it pulls the ocean
with more g force than the sun?
 No. Tides are not caused by gravitational pulls but
rather differences in the gravitational pull. If the moon
was closer, the difference in pulls would be too much
and it would tear the moon to pieces. This is what
some think caused the rings of Saturn
Presented by April Senger
Full Moons?
 During a new or full
moon the Sun, Earth
and moon are all
lined up
 This causes the
highest tides called
spring tides (nothing
to do with the
season)
 When the moon is
half way through a
phase, the Sun and
moon’s pulls partially
cancel each other
producing a neap
tide
 Why don’t lakes have
tides?
Presented by April Senger
Good Tidings for All
 Tides can occur in the atmosphere and
magma called Earth Tides
 For the purpose of this class we ignore
the tilt of the Earth, landmasses, friction
with ocean bottoms, and other factors
that influence tidal motion
 It is a complex and fascinating
occurrence
Presented by April Senger
Gravity and the Universe
 We will spend more time studying the origin of the
Universe in another chapter but consider this…
 If everything that has mass has gravity, won’t we all
eventually pull ourselves back into one central
location?
 Some believe in a theory called the Big Bang but in the
future we might experience the Big Crunch when all
material centralizes
 Scientists that believe in the Oscillating Universe
theory feel that it would take about 100 billion years for
the process to occur
Presented by April Senger