#2330—News Summary


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.


Stand two large cups that are the same size on a flat surface about 20 cm apart.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.


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?


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.


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.”


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.


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?


Tie a piece of string1.    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.


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.”


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.


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.


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.

pour water into the bottle almost to the top7.    Have another assistant pour water into the bottle almost to the top.


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!


Dr. Joanne HillDr. 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.

See ashort clip (requires the free RealPlayer plug-in)



Do you know…

  1. the four U.S. states, three Canadian provinces, and two bays that border the province of Quebec?
  2. the ten U.S. presidents who have held office while Fidel Castro has held power in Cuba?
  3. the capital city of Benin?
  4. the street address of the White House?
  5. the types of energy on either side of visible light on the electromagnetic spectrum?
  6. how much you would weigh if you were in a 2-g environment?

(See the answers in next week's GO FIGURE!)

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