# 13 Universal Gravitation

13 Universal Gravitation
Instructions on using this
PowerPoint:
• Please review this PowerPoint while
simultaneously completing the EXERCISES:
Weight, Weightlessness, and Gravity Notes.
• I would recommend skimming the pages in your
textbook that correspond with your notes; they
are labeled on your Notes (p.186, p. 129-132)
the PowerPoint – challenge yourselves!
13 Universal Gravitation
13 Universal Gravitation
13.3 Weight and Weightlessness
Pressure against Earth is the sensation
we interpret as weight.
13 Universal Gravitation
13.3 Weight and Weightlessness
•The force of gravity, like any force, causes acceleration.
•Objects under the influence of gravity are pulled toward each
other and accelerate.
•We are almost always in contact with Earth, so we think of
gravity as something that presses us against Earth rather than
as something that accelerates us.
#1:
because we are always in contact with Earth…we are used to
it!
13 Universal Gravitation
13.3 Weight and Weightlessness
Stand on a bathroom scale that is supported on a stationary
floor. The gravitational force between you and Earth pulls you
against the supporting floor and scale.
By Newton’s third law, the floor and scale in turn push
upward on you.
#2 –
This pressure against Earth’s surface is the sensation we
interpret as weight.
13 Universal Gravitation
13.3 Weight and Weightlessness
The sensation of weight is equal to the force that you exert
against the supporting floor.
-If you stand on the scale, your weight is “normal”
-When the elevator accelerates upward, the bathroom scale
and floor push harder against your feet. The scale would show
-#3
B
-#5
C
13 Universal Gravitation
13.3 Weight and Weightlessness
•When the elevator accelerates downward, the support force of
the floor is less. The scale would show a decrease in your
weight.
•If the elevator fell freely, the scale reading would register zero.
According to the scale, you would be weightless.
You would feel weightless, for your insides
would no longer be supported by your legs.
**Remember – weightlessness
is really a LACK of SUPPORT FORCE
#4
D
#6
A
13 Universal Gravitation
13.3 Weight and Weightlessness
#7 Rather than define your weight as the force of gravity
that acts on you, it is more practical to define weight as the
force you exert against a supporting floor; OR support
force! According to this definition, you are as heavy as you
feel.
#8 –
False
# 9:
The condition of weightlessness is not the absence of gravity,
but the absence of a support force.
13 Universal Gravitation
13.3 Weight and Weightlessness
Look at the picture:
Both people are without a
support force and therefore
experience weightlessness.
**Astronauts still have
GRAVITY acting on
them…they just lack a
SUPPORT FORCE**
13 Universal Gravitation
13.3 Weight and Weightlessness
What sensation do we interpret as weight?
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
From within a rotating frame of reference, there
seems to be an outwardly directed centrifugal force,
which can simulate gravity. *It does NOT create
gravity…it just FEELS like it*
# 10 –
The centrifugal force experienced in a rotating
reference frame is not a real force.
#11 –
Real forces include: gravitational, nuclear, &
electromagnetic.
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
Consider a colony of ladybugs living inside a bicycle tire.
If the wheel falls through the air, the ladybugs will be in a
weightless condition and seem to float freely while the wheel is
in free fall.
***This is similar to astronauts in space
If the wheel is spinning, the ladybugs will feel themselves
pressed to the outer part of the tire’s inner surface. At the right
spinning speed, the ladybugs will experience simulated gravity.
***This is similar to a hypothetical rotating space station –
if it is large enough, and spun at the right speed, it
SIMULATES the FEELING of Earth gravity.
#12 –
true
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
Gravity is simulated by centrifugal force.
#13 –
To the ladybugs, the direction “up” is toward the center of
the wheel, or the center of rotation.
The “down” direction to the ladybugs is what we call
“radially outward,” away from the center of the wheel.
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
Need for Simulated Gravity
Today we live on the outer surface of our spherical planet,
held here by gravity.
What happens if we can no longer inhabit Earth?
In the future, people may live in huge, slowly rotating
space stations where simulated gravity allows them to
function normally.
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
Support Force
Occupants in today’s space vehicles feel weightless
because they lack a support force.
Future space travelers need not be subject to
weightlessness.
Their space habitats will probably spin, effectively supplying
a support force and simulating gravity.
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
The man inside this rotating space habitat experiences
simulated gravity.
• As seen from the outside, the only force exerted on the
man is by the floor.
• The floor presses against the man (action) and the
man presses back on the floor (reaction).
• The only force exerted on the man is by the floor.
• It is directed toward the center and is a centripetal
force.
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
As seen from inside the rotating system:
• In addition to the man-floor interaction, the man FEELS a
centrifugal force pressing him on the wall. It seems as
real as gravity.
• Yet, unlike gravity, it has no reaction counterpart.
• Centrifugal force is not part of an interaction, but results
from rotation. It is therefore called a fictitious force.
• Centrifugal force is actually the acceleration you feel from
rotation
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
Challenges of Simulated Gravity
The comfortable 1 g we experience at Earth’s surface is due
to gravity.
Inside a rotating spaceship the acceleration experienced is
the centripetal acceleration due to rotation.
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
Small-diameter structures would have to rotate at high speeds
to provide a simulated gravitational acceleration of 1 g.
Sensitive and delicate organs in our inner ears sense rotation.
Although there appears to be no difficulty at 1 RPM, many
people have difficulty adjusting to rotational rates greater than
2 or 3 RPM.
*** Remember the ‘Vomit Comet’ video?**
To simulate normal Earth gravity at 1 RPM requires a large
structure—one almost 2 km in diameter.
13 Universal Gravitation
9.5 & 9.6 – Centrifugal Force & Simulated Gravity, pgs 129-132
This NASA depiction of a rotational space colony may be a
glimpse into the future.
#14 –
Rotation will allow people inside the space station to
experience a support force that simulates normal Earth gravity.

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