WEBVTT

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- A MOON ORBITING A PLANET,

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A SPINNING FERRIS WHEEL,

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A YO-YO TRICK
CALLED AROUND THE WORLD,

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THE INTERNATIONAL SPACE STATION:

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THEY'VE ALL GOT
AT LEAST ONE THING IN COMMON.

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FIND OUT HOW THEY ALL COME
FULL CIRCLE,

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NEXT ON "REAL WORLD."

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A MOON ORBITS A PLANET

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JUST LIKE A PLANET
ORBITS A STAR:

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IN A CURVED,
NEARLY CIRCULAR PATH.

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A FERRIS WHEEL CURVES
AROUND A CENTRAL POINT AS WELL.

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AND, OF COURSE, WHEN YOU GO
AROUND THE WORLD WITH A YO-YO,

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IT'S CIRCLING
AROUND YOUR HAND.

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AND WHY
DOES ALL OF THIS HAPPEN?

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BECAUSE OF A LITTLE THING
CALLED CENTRIPETAL FORCE.

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BASICALLY, CENTRIPETAL FORCE
IS A FORCE THAT MAKES AN OBJECT,

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LIKE PLANETS
OR OTHER SATELLITES IN SPACE,

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FOLLOW A CURVED PATH.

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THE WORD CENTRIPETAL MEANS
"CENTER-SEEKING."

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WHAT BETTER PLACE
TO STUDY CENTRIPETAL FORCE

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THAN ON THE LARGEST
ARTIFICIAL SATELLITE IN SPACE,

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THE INTERNATIONAL SPACE STATION?

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IN ADDITION TO ACTUALLY
EXPERIENCING CENTRIPETAL FORCE,

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THE FREE-FALL ENVIRONMENT
ON BOARD THE ISS

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ALLOWS FOR SOME REALLY COOL
DEMONSTRATIONS.

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SO LET'S SEND IT UP
TO EXPEDITION 22 ASTRONAUT

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JEFF WILLIAMS.

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- THIS IS A TOOL.

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IT'S A SPECIAL TOOL THAT WE USE
ON BOARD THE SPACE STATION,

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ACTUALLY, DURING SPACE WALKS.

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BUT IT'S A HEAVY METAL OBJECT,

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AND THAT'S WHAT I REALLY
WANTED TO USE.

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AND YOU CAN SEE I HAVE IT TIED
TO A STRING--

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OR WITH A STRING
ONTO THIS BUNGEE RIGHT HERE.

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AND IT'S JUST FLOATING HERE
IN WEIGHTLESSNESS.

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AND YOU CAN SEE THE STRING HERE
IS RATHER LOOSE,

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AND IT'S JUST KIND OF
FLOATING RANDOMLY.

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IF I WERE TO ROTATE THIS THING

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ON THE STRING
AROUND THIS BUNGEE--

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AND LET ME DO THIS--

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YOU CAN SEE THAT THE STRING
PULLS TAUT AND STAYS TIGHT

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AND THE TOOL CONTINUES TO ROTATE
AROUND THE BUNGEE.

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IN FACT, IF YOU LOOK
REALLY CLOSE AT THE BUNGEE,

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YOU CAN SEE
THAT THE BUNGEE BENDS

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AT THE POINT
THAT IT'S ROTATING,

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AND IT BENDS TOWARD THE TOOL.

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AND THAT IS CAUSED
BY THE CENTRIPETAL FORCE

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DUE TO THE ANGULAR ACCELERATION
OF THE TOOL

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AS IT ROTATES AROUND,

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KEEPING THIS STRING TIGHT

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AND KEEPING THE ROTATION
IN A CIRCULAR MOTION.

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IT'S THE SAME KIND OF FORCE

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THAT APPLIES TO THE ROTATION
OF PLANETS AROUND THE SUN

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OR THE MOON AROUND THE EARTH

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OR THE SPACE STATION
AROUND THE EARTH.

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- BUT YOU MIGHT NOTICE

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THAT NEITHER THE MOON
NOR THE ISS

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IS ATTACHED TO EARTH
BY A STRING,

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AND THEY STILL ORBIT.

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WHY IS THAT?

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WELL, THINK ABOUT THIS.

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SATELLITES WOULD CONTINUE
TO MOVE IN A STRAIGHT LINE

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BECAUSE OF INERTIA.

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REMEMBER THAT INERTIA
IS THE TENDENCY OF AN OBJECT

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TO KEEP ON DOING
WHAT IT WAS DOING

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BEFORE SOMETHING
FORCED IT TO CHANGE.

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GRAVITY IS THE FORCE
CAUSING THAT CHANGE.

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SATELLITES IN ORBIT
AROUND EARTH

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ARE BEING PULLED TOWARD EARTH
BY EARTH'S GRAVITY.

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ACCELERATION OCCURS
BECAUSE GRAVITY

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IS CONTINUOUSLY CHANGING
THE SATELLITE'S DIRECTION.

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TO UNDERSTAND HOW THIS WORKS,
TAKE A LOOK AT THIS.

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I'VE GOT A YO-YO, AND I'M GOING
TO SPIN IT AROUND MY FINGER

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ON THIS TABLE.

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THE STRING HERE IS ACTING
LIKE THE FORCE OF GRAVITY.

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IF I LET GO WHEN THE YO-YO
IS AT THIS TOP POINT,

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WHERE DO YOU THINK THE YO-YO'S
INERTIA IS GOING TO TAKE IT?

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WILL IT CONTINUE
IN A CIRCULAR MOTION?

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AS YOU CAN SEE,
THE YO-YO WANTS TO KEEP MOVING

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IN A STRAIGHT LINE
INSTEAD OF A CURVED ONE.

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THAT'S INERTIA.

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THE STRING IS THE ONLY THING

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THAT HELD THE YO-YO
IN ITS CIRCULAR PATH.

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THE STRING SIMULATES GRAVITY.

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IN THE SAME WAY, THE ISS IS HELD
IN ORBIT BY EARTH'S GRAVITY.

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IF, FOR SOME STRANGE REASON,

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EARTH'S GRAVITATIONAL PULL
DISAPPEARED,

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THE ISS WOULD CONTINUE

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IN WHATEVER DIRECTION
IT WAS HEADING.

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HERE'S JEFF
WITH ANOTHER DEMONSTRATION

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OF CENTRIPETAL FORCE.

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- OKAY, I'VE GOT THIS TEA BAG,

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AND IT'S GOT TEA IN IT,
OF COURSE,

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AND IT'S GOT AIR BUBBLES
FLOATING AROUND HERE.

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THE BUBBLES
DON'T RISE TO THE TOP

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IN THE ABSENCE OF GRAVITY,

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IN THIS WEIGHTLESS ENVIRONMENT
OF THE SPACE STATION.

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BUT IF I WERE TO ROTATE THIS--

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AND LET'S SEE
IF I CAN ROTATE IT

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AND KEEP IT IN YOUR VIEW
OF THE CAMERA.

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LET'S WATCH WHAT HAPPENS.

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IF YOU LOOK REALLY CLOSE,
YOU'LL SEE THE AIR BUBBLES

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STARTING TO COALESCE
IN THE CENTER OF THE TEA BAG.

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I'M GOING TO TRY IT
ONE MORE TIME.

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THERE'S THE BUBBLES
SPREAD THROUGHOUT THE TEA.

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AND IF I ROTATE IT...

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THE BUBBLES COALESCE
TO THE CENTER OF THE TEA

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AND EVENTUALLY FORM A CIRCLE.

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WHY DO THEY DO THAT?

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BECAUSE OF THE CENTRIPETAL FORCE
SPREAD THROUGHOUT THE TEA,

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THE LIQUID TEA GOES
TO THE OUTSIDE OF THE BAGS,

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AND IT FORCES THE AIR
INTO THE CENTER

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BECAUSE THE AIR HAS LESS MASS--
LESS DENSITY THAN THE LIQUID,

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SO THE AIR GOES TO THE CENTER.

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- CENTRIPETAL FORCES
AFFECT THINGS HERE ON EARTH,

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AS WELL AS IN SPACE--

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THINGS LIKE FERRIS WHEELS

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OR THE ACCELERATION OF A CAR
GOING AROUND A CURVE--

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BUT WHAT OTHER APPLICATIONS
MIGHT THIS FORCE HAVE IN SPACE?

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- YOU'VE HEARD A LOT OF TALK,
PERHAPS,

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IN REGARDS TO SPACE EXPLORATION
ABOUT ARTIFICIAL GRAVITY.

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PERHAPS YOU'VE SEEN
SOME SCIENCE FICTION MOVIES,

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FOR EXAMPLE, "STAR TREK,"
THE TV SHOW AND THE MOVIES.

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THEY HAD GRAVITY ON BOARD.

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THEY HAD SOME KIND OF
A MYSTERIOUS ARTIFICIAL GRAVITY,

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MAKE-BELIEVE, OF COURSE.

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BUT WE'VE TALKED FOR YEARS

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ABOUT ARTIFICIAL GRAVITY
ON BOARD A SPACECRAFT,

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ESPECIALLY WHEN YOU'RE GOING
FOR A LONG PERIOD OF TIME,

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FOR VARIOUS REASONS.

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ONE OF THE WAYS YOU COULD
PRODUCE ARTIFICIAL GRAVITY

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IS BY HAVING A SPACE STATION
THAT ROTATES,

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MAYBE VERY BIG,
AND THE BIGGER IT IS,

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THE SLOWER THE ROTATION
WOULD HAVE TO BE

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TO PRODUCE
THAT CENTRIPETAL FORCE.

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- JEFF SAID THAT THE BIGGER
THE STATION,

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THE SLOWER IT WOULD HAVE TO SPIN

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TO CREATE
THE CENTRIPETAL FORCE NEEDED

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TO MAKE THAT ARTIFICIAL GRAVITY.

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TO FIND OUT WHY,
WE HAVE TO CHECK OUT THE MATH.

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CENTRIPETAL ACCELERATION
IS EQUAL

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TO THE VELOCITY OF THE OBJECT
SQUARED

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DIVIDED BY THE RADIUS
OF THE OBJECT.

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SPIN IS VELOCITY
DIVIDED BY RADIUS.

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THE RADIUS IS THE DISTANCE
FROM THE CENTER OF THE CIRCLE

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TO ANY POINT ON THE CIRCLE.

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THE SPIN AND SIZE
OF THE RADIUS

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ARE ACTUALLY
AN INVERSE PROPORTION.

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ON A SPINNING STATION,
THE LARGER THE RADIUS,

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THE SMALLER THE RATE OF CHANGE
NEEDED TO SIMULATE GRAVITY.

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- THEORETICALLY,
YOU COULD DO THAT.

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THEORETICALLY, YOU COULD TAKE
A LARGE STATION

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AND PRODUCE,
THROUGH CENTRIPETAL FORCE,

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SORT OF AN ARTIFICIAL GRAVITY
TO KEEP EVERYTHING IN ITS PLACE.

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BUT THE PROBLEM WITH THAT
IS WHEN--

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IT'S NOT VERY PRACTICAL,

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BECAUSE WHEN YOU SPIN SOMETHING

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AND WE TURN OUR HEAD AROUND,
OUR HEAD--OUR VESTIBULAR SYSTEM

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WILL GET A LITTLE BIT MESSED UP.

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AND IT'S LIKE IF YOU'RE GOING
AROUND ON A MERRY-GO-ROUND,

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RIDING A MERRY-GO-ROUND,
AND THEN YOU STEP OFF THE THING

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OR YOU SPIN AROUND
UNTIL YOU GET DIZZY.

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IT'D BE THE SAME KIND OF THING.

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SO I'M NOT SURE
ARTIFICIAL GRAVITY

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IN THAT WAY
IS VERY PRACTICAL.

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- WONDER WHO WILL BE THE FIRST
TO PRODUCE ARTIFICIAL GRAVITY?

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BUT HEY, DON'T JUST SIT THERE
AND SPIN YOUR WHEELS.

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USE THAT MATH TO HELP SOLVE
THIS CHALLENGE.

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I'LL SEE YOU NEXT TIME
ON "REAL WORLD."