Mathematics/Physics

 

Center of Gravity

 

Wanda Pitts

Douglas Community Academy

3200 S. Calumet

CHICAGO IL 60616

 

(773) 534-9263

 

Objective(s):

 

Third grade students will be able to find the center of gravity and to define force and gravity.

 

 

Materials:

 

Assorted shapes made of cardboard, yarn, washers; push pins, and a cork bulletin board.

 

A knife, new pencil sharpened, potato, empty 2-liter soda bottle with cap, and two metal forks of equal size.

 

Strategy:

 

Ask the question what is force? A force is a push or pull. The teacher will put the definition on the board. See if you can find the center of gravity of an irregular shape.                                                           

      1. Cut out the shape from a piece of cardboard.     

      2. Pin the shape to a bulletin board with yarn and attach a weight at the end of the yarn.

      3. Draw a line behind the yarn, pin the shape at several different points and draw a line each    

           Time.            

      4. The point at which the lines cross is the center of gravity of your shape.

  Can you balance the shape from this point on the eraser of a pencil?  

 

Ask the question what is gravity?  The teacher will put the definition on the board.  Gravity is the force that pulls objects back toward the earth.  Gravity is the pulling force between two objects due to their masses.

  1. The teacher will cut a 1-inch thick slice from the center of a potato.  Push the pencil through the center of the potato.
  2. Place the capped bottle on a desk.  Try to balance the pencil (on its eraser) on the top of the cap.  
  3. Stick the two forks into opposite sides of the potato.
  4. Now try balancing the pencil (on its eraser) again on top of the soda bottle.  Adjust the forks slightly if needed.

What happened?  All things have a balance point, or a center of gravity, at which they are in perfect balance.  The pencil with the potato is top heavy and will not balance.  By adding forks to the potato, you change the center of gravity.

                                                                                                                                                               

 

Performance Assessment:

 

The students should be able to use specific vocabulary to define force and gravity.

The students should be able to find the center of gravity of an object through the demonstration of these experiments.

 

Conclusions:

 

Every weight has a center of gravity.

 

References:

 

McGraw Hill Science Teacher’s Edition, New York, New York, 2000

 

Taylor, Barbara; Weight and Balance, New York 1990