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Gravity is the attractive force between all objects in the universe. It is the force that pulls objects to Earth. It is the force that keeps all of the planets in orbit. Would you like to know how it feels to be in control of this great force? Hey, let’s simulate the gravity! Grab a partner and a tennis ball. Find some string and firmly attach the string to the ball. You could tie the string to the ball, use a fish hook swivel, or even a wall anchor/molybolt. Be creative if you need to be, what ever works.
 

Materials:

  • One ball
  • Ten inch piece of string
  • Data table
 
Safety concerns: Be sure to keep all eye safety rules. As with all science lab activities, the most important safety rule is to follow all teacher directions.
 

Procedure:

  1. In this activity there are two jobs, a scribe and a twirler. Each person will have the opportunity to do both jobs.
  2. Firmly attach the string to the ball, using whatever method you have chosen.
  3. Once you have your string attached, you will measure 30 cm from the ball and make a mark on the string with a marker. Make another mark at 120 cm, and the last mark at 270 cm.
  4. Twirler-
    • Stand in an open area with at least 3 meters of space between you and anything or anyone else.
    • Tightly grasp your string at the 30 cm mark and twirl it around your head.
    • Tell the scribe how your hand/arm is moving right now and how hard you have to pull.
  5. Scribe-
    • Write the observations as the twirler explains them to you.
  6. Twirler, now let the string slide through your fingers, while twirling release the string until your hand is on the 120 cm mark. Grasp firmly and repeat steps four and five.
  7. Twirler, now let the string slide through your fingers, while still twirling release the string until your hand is on the 270 cm mark. Grasp firmly and repeat steps four and five.
  8. Switch jobs and repeat the entire exercise.
 

Data:

Copy the following data table in your science journal (or print it as a .pdf document.)

Fly Ball Data Table
Name
Hand/Arm Motion
How hard did you pull?
     
     
 
 

In this activity, the string represents the force of gravity. Each time you twirled the ball there had to be enough oomph to keep the ball from falling out of orbit. If the string couldn’t keep the ball up easily, the twirler had to supply the difference. To put it simply, what the string couldn’t do, you had to add yourself. The more you do the weaker the string was.

Now that you have both had the opportunity to twirl, decide at which length you had the most control. Got your answer???

Highlight the box below to check your answer!
Hopefully, you chose the shortest length. At the shortest length the force of gravity was the strongest as evidenced by the fact that it was easier to control. However, think about how difficult it was to control the ball and keep its momentum at 270 cm. This implies that the gravitational force becomes weaker as distance increases.
 
As you may have discovered in the activity, distance can have an affect on gravity. Thinking about this, if you were an astronaut, and you go farther into space, why do you become weightless? Well, as your distance from Earth increases, the force of gravity between you and Earth decreases. So when you're in space, gravity has less effect on your mass. Look closely at the following diagram to help you understand this concept.

The closer together two objects are, the greater the gravitational force.

The greater the distance between two objects, the weaker the gravitational force.

 
 
Analysis:
  1. What is the relationship between distance and the force of gravity?
  2. You are feeling the force gravity from both the Earth and the Moon, so why don’t you feel the Moon’s pull?
  3. Could you ever become truly weightless in our universe?

Review Science safetey rules here.

Get the plug-ins: Get Adobe Acrobat Reader , and Get Quicktime Player. (The QuickTime plug-in is needed to play sounds and movies correctly.)

Want to share photos of you or your friends doing this activity? Send it in an e-mail with the following information:

  1. The title of the activity
  2. The URL (Internet address)
  3. Your name.

Remember that no pictures can be used that show student faces or student names on it. 

Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 8th grade science core.


Updated October 24, 2008 by: Glen Westbroek

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