NSF-GK12 Program at Chelsea High School

Physics

Physics of gravity

This lesson plan describes a demonstration that seems to "defy gravity" while at the same time illustrates the concept of center of gravity.

The purpose of this lesson is to introduce and illustrate the concept of center of gravity and to enable students to predict center of mass of objects.

1. A 16 oz carpenter's hammer
2. A wooden 12 inch ruler
3. About 10 inches of string
4. Small piece of tape (any kind)

Set-up:
Make a small loop with about 10 inches of string that is strong enough to hold the weight of the hammer. Slip this loop around the handle of the hammer (if the handle is polished, you may need to tape the string in place around the handle to prevent sliding). Next, slide the ruler through this same loop. Put the "head" of the hammer towards the 1" mark end on the ruler.  Depending on the size of your hammer, the string should rest between the 3" and 5" mark on the ruler. The edge of the handle should rest against the wooden ruler. The ruler and handle should form an angle of about 30 to 45 degrees. It is important that the head of the hammer extends beyond the edge of the ruler - an inch beyond the edge of the ruler should be more than enough. Now you should be able to carefully balance the device off the edge of a table. The hammerhead should be under the table surface that the ruler is resting on. With a little practice, you will be able to balance this heavy, seemingly awkward device with only one 16th of an inch or less in contact with the table. This works because the center of mass (somewhere near the hammerhead) is under the supporting end of the ruler on the tabletop. It looks impressive because the majority of the ruler and handle is suspended off the edge of the table.

See the diagram of this demo here.

In Class:

Demonstrate the ruler-hammer system described above to the class.  Ask students what is awkward about this system.  Ask them whether you are defying the gravity.  Guide the class discussion towards whether it is possible to defy gravity.  By giving different examples, establish that it is not possible to defy gravity.  So, there must be a logical explanation for this system.  Write on the board “What is Center of gravity?”

Let students brainstorm and guess what a center of gravity is.  Guide the class discussion by asking where on an object gravity pulls.  Give sphere objects as examples.  Start with drawing a very small sphere (circle) on the board and ask where gravity applies on this object.  Then draw a bigger circle and ask the same question.  Next draw an even bigger circle and ask the same question.  Repeat this procedure with a small box example.  Now, refer back to the question, “what is center of gravity?” 

Write the definition of center of gravity on the board, the point where gravity can be said to act.  The center of gravity of a system of particles is a specific point at which the system's mass behaves as if it were concentrated.

Continue the discussion by drawing two spheres connected by a bar on the board.  Ask the students where they think the center of gravity is if the spheres have the same mass.  If I added one more sphere (mass) to the top of the left sphere, does the center of gravity of the system shifts to the left, right, or stays at the center?  Guide the students to get the right answer.  If there are students who have hard time to guess the correct answer, ask them to find the center of gravity of their pens by balancing them on their index fingers and what will happen if they added more weight to the left side of the pens. 

Continue with the example on the board, and ask students what will happened to the center of gravity of the system as you add more mass to the left sphere.  Now, go back to the demo, and ask students what they think the explanation is for this awkward looking system.  Guide the class discussion towards the fact that the center of gravity of the system is right at the point that the ruler touches the table.  The teacher can even balance the system at that point on his finger.

Conclusion:

All objects have a balance point, or a center of gravity, at which they are in perfect balance.

Have the studetns to predict and find the center of gravity of different objects.

NSTA, University of Northern Colorado.  http://mast.unco.edu/hotline/index.php