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FUN
WITH GRAVITY!
Which
way is down?
On
Earth, that’s usually an easy question to answer,
thanks to gravity. Gravity is the invisible force that pulls
objects toward one another. The strength of the pull depends
on how massive an object is.
Earth
is very massive, so its gravity is very strong. It pulls
everything on or near its surface to the center of the planet.
SEE
FOR YOURSELF!
 1.
Stand two large cups that are the same size on a flat surface
about 20 cm apart.
2.
Place a ruler on top of the cups so that it spans the distance
between them.
3.
Tie a short piece of string (about 25 cm) to a paper clip.
4.
Tie the other end of the string around the center of the
ruler so that the paper clip does not touch the surface.
OBSERVE:
Which
way does the hanging paper clip point? Draw a sketch in
your Gravity Notebook to show the position of the paper
clip.
Raise
one end of the ruler about 5 cm above the top of the cup.
Draw a sketch of the paper clip’s position.
Now
raise the other end of the ruler about 10 cm above the top
of the cup. Draw a sketch showing the paper clip’s
position.
Compare
the three sketches. What can you conclude? Does the position
of the ruler affect which way the paper clip points?
Does
the weight of an object affect the speed at which it falls?
SEE
FOR YOURSELF!
1.
Choose two balls of similar size, but different weights,
such as a ping-pong ball and a squash ball.
2.
Place a metal cookie sheet on the floor.
3.
Hold one ball in each hand shoulder-high over the cookie
sheet.
4.
At the exact same time, release both balls.
5.
Listen for the sound of the balls hitting the cookie sheet.
6.
Repeat the ball release activity several times.
OBSERVE:
Does
one ball consistently hit the cookie sheet first, or do
they both consistently hit at the same time? Record each
trial in your Gravity Notebook.
Now
try standing on a chair and repeating the experiment. Does
the increased distance make a difference?
What
can you conclude about the speed of falling objects?
Repeat
the experiment one more time. This time, roll a lump of
modeling clay into a smooth, flat sheet and fit it inside
the cookie sheet. Now…
1.
Drop the balls onto the clay-line sheet.
2.
If the balls haven’t bounced off, carefully remove
them from the clay.
3.
Examine the dent made in the clay by each ball.
4.
In your gravity notebook, describe the impression made by
the light ball and the impression made by the heavy ball.
What
can you conclude? Which ball pushes through the air with
more force?
Why
doesn’t gravity pull the space shuttle back to Earth?
Astronauts
stay in orbit for the same reason that you stay crammed
in your roller coaster seat even when you’re on an
upside down loop. Gravity is pulling on you, but a curved
path can create another force that can work against gravity.
The force is known as centrifugal, which is Latin for “flying
away from the center.”
SEE
FOR YOURSELF!
1.
Take a paddle with a ball attached to a rubber band and
swing it slowly around your head. It will wobble and flop
downward as gravity pulls on it.
2.
Gradually increase the speed at which you swing the ball.
The centrifugal force will get stronger - strong enough
to keep the ball in “orbit” around you!
Now
try this:
1.
Hold the paddle in one hand and the ball in the other at
shoulder height, stretching the ball as far as the rubber
band will allow.
2.
Release the ball.
3.
Repeat the activity, this time holding the ball above your
head while keeping your arms stretched and the paddle at
shoulder height.
OBSERVE:
In
your gravity notebook, sketch the path that the ball takes
each time. Is there another force besides gravity at work?
What can you conclude about the path of the ball as it falls?
How
does free-falling affect weight?
A
space shuttle doesn’t ever travel beyond Earth’s
gravity. The force of gravity holds the shuttle in orbit.
But the speed of the shuttle creates a centrifugal effect
that balances the pull of gravity. The shuttle is actually
in a free-fall. But the spaceship doesn’t crash into
Earth because it falls in an arc that curves in the same
direction as Earth’s surface, just as the ball returned
to the paddle in a curved path. What about the astronauts?
SEE
FOR YOURSELF!
 1.
Tie a piece of string (about 30 cm) around crayon.
2.
Place an empty 2-liter soda bottle on the floor.
3.
Put the crayon into the bottle, being careful to hold onto
the other end of the string.
4.
Raise the string slowly so that the crayon is horizontal
inside the bottle.
5.
Using the string, pull the bottle up until it is at shoulder
height.
6.
Station an observer on the floor to watch the crayon.
7.
Release the string and let the bottle fall to the floor.
OBSERVE:
What
was in free-fall — the bottle, the crayon inside the
bottle? Or both? What can you conclude about the astronauts
on the space shuttle? If you were on a scale that was on
an elevator and the elevator cable suddenly snapped plunging
you into a free-fall, would your weight change? How much
do you think you would weigh?
How
can we determine the gravitational attraction of an object?
Your
weight is actually a measure of the force of gravity between
you and the earth. So another way to describe weight is
“gravitational attraction.”
SEE
FOR YOURSELF!
1.
Tape the end coil of a plastic Slinky to the top of a doorframe,
about 1-cm from the side of the doorframe.
2.
Punch holes in opposite sides of a 5-ounce paper cup.
3.
Tie each end of a short piece of string (25 cm) to one of
the holes to make a string handle for the cup.
4.
Hang the string handle onto the last coil at the bottom
of the Slinky.
5.
Tape a sheet of plain white paper to the side of the doorframe
beside the Slinky.
6.
Tape a short pencil (about 5 cm) to the side of the cup
so that the point faces the paper.
7.
Using a marker, make a short, horizontal line on the paper
at the place that the pencil is pointing to, and label it
“0.”
8.
Drop a quarter into the cup. Make a line to show the pencil’s
new position and label it “1.”
9.
One by one, drop up to six quarters in the cup, each time,
marking the new position of the pencil and noting the number
of quarters.
OBSERVE:
Remove
the paper from the wall and measure the distance from one
horizontal line to the next. Could you have predicted where
the pencil would have pointed if you had added a seventh
quarter?
You
have just made a spring scale! What can you conclude about
how a spring scale works?
Because
planets that are more or less massive than Earth have different
gravity forces, your weight would change from one planet
to another. Your size wouldn’t change because your
body mass (your skin, bones, and all the other particles
that make up your body) would still be the same. The only
way to change your mass is to add more particles - just
as you added more quarters to the spring scale.
How
do astronauts prepare for working in microgravity?
For
creatures that have never ventured outside Earth’s
gravity, the sensation of weightlessness can be very strange!
Astronauts practice for space missions in a huge tank of
water known as the neutral buoyancy lab. Buoyancy is the
upward force that water exerts on an object, and it can
balance some of the downward force of gravity.
SEE
FOR YOURSELF!
1.
Cut the bottom from an empty 2-liter soda bottle. (Because
you’ll need a sharp object as your cutting tool, you
might want to ask an adult to do the cutting for you!)
2.
Take a piece of yarn or cord (about 50 cm) and insert it
into the bottle so that a short piece (about 5 cm) is hanging
out of the mouth of the bottle.
3.
Screw the bottle cap on so that it holds the short piece
of yarn securely in place.
4.
Thread the other end of the yarn (which is now inside the
bottle) through 4-6 empty thread spools that are all the
same size.
5.
Turn the bottle upside down so that the cap rests on paper
towels on a flat surface. Have an assistant hold the bottle
in place.
6.
Gently raise the other end of the yarn until all the spools
are in a straight column.
 7.
Have another assistant pour water into the bottle almost
to the top.
OBSERVE:
In
your Gravity Notebook, sketch a picture of the spools without
water and a picture of the spools after the water has been
added. What differences do you note? Does water increase
or reduce the effects of gravity? What can you conclude
about using a pool of water to train for a low gravity environment?
In
microgravity, an astronaut’s spinal cord behaves like
the spools do in water. Without gravity to pull them down,
the discs that make up the spinal cord may move apart, so
an astronaut may be taller in orbit than on the ground!
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Dr.
Joanne Hill is an astrophysicist who is part of a gamma
ray hunting team. When NASA launches the Swift Gamma Ray
Explorer in 2003, she hopes to peer into the hearts of black
holes, antimatter, and dying stars. We asked Dr. Hill what
subject she liked best in elementary school and how it helped
her to become a gamma ray hunter.