| FUN
WITH GRAVITY!
According
to legend, Isaac Newton made his observations on gravity after
watching an apple fall from a tree. Make your own observations
by doing the following experiments and recording the results in
a gravity notebook!
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?
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?
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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