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Gravity Simulation Activity

Materials

• 1 -- 3/4 in. thick - 6 x 10 in. wood base platform
• 2 -- 3/8 in. dowel - 8-1/2 in. long post support
• 2 -- 1/8 in. dowel - 9 in. long horizontal support
• 9 -- 3/4 in. diameter - 1/4 in. thick round magnets with 1/4 in. center hole

Stand Assembly Preparation

1. Drill 2 3/8 in. holes 8 in. apart for the post supports.
2. Sand one end of each post support dowel until it just fits smugly into the base platform.
3. In each post support dowel, drill 3 5/32 in. holes: the 1^st hole being 1 in. from the top of the post support dowel; the 2^nd hole 1-1/2 in. below the 1^st; the 3^rd 1-1/2 in. below the 2^nd hole.
4. Drill a 4^th 5/32 in. hole halfway between the 2^nd and 3^rd holes.

Helpful Hints:

1. Once you get the post support dowels lined up so that the horizontal support dowels fit snuggly but easily, mark the post dowels and base in such a way that after you disassemble the apparatus you will be able to put it back together so everything lines up again.
2. Coat the horizontal supports with graphite (#2 pencil lead works great) to help magnets move smoothly.

Procedure

1. Consider each magnet to be 1 gravitational unit.
2. Start out by having 3 magnets linked together on a horizontal support in the top holes of the post supports. These magnets represent a random "celestial body".
3. Place 6 magnets linked together on a horizontal support through the next set of holes. These six magnets represent the mass of the Earth.
4. Align the top magnets so they are pushed as the magnets on the lower horizontal support are moved towards them. Place a finger in the path of the top magnets to feel the force pushing on them. Record and discuss your observations.
5. Reverse the polarity of the top "celestial body" so it is now pulled by the lower one instead of being pushed. Do you feel any difference between the push and pull of gravity?
6. Remove 5 magnets from the "earth" on the bottom horizontal support. The new "celestial body" now has 1/6 the mass of the "earth" and becomes the "earth's moon".
7. Repeat Step 4 and record and discuss your observations.
8. Restore the bottom "celestial body" to 6 mass units and place its horizontal support in the bottom post support holes. Repeat Step 4; record and discuss your observations.
9. Move the horizontal support with the 6 magnets up to the second hole from the bottom and repeat Step 4. Again, record and discuss your observations.
10. Keeping the bottom horizontal support in its present holes take one magnet from the top support and add it to the six. Repeat Step 4. The top "celestial body" is now moved by the bottom one yet the combined mass of the two "celestial bodies" has not changed nor has the distance between them. Why then is the top "celestial body" affected by the bottom one?
11. Newton found that the gravitational force between two objects is proportional to the inverse of the separation squared (F ∞ 1/r²) and applies to any two masses no matter what their size. The mathematical equation for Newton's Universal Law of Gravity thus becomes F = Gm1m2/r², where G is called the Gravitational Constant and has a value of 6.67 x 10^-11 Nm²Kg^-2. When the m values are in kilograms and r is in meters F (the force) will be expressed in Newtons.
12. Using Newton's Universal Law of Gravity, quantify what has just taken place with your "celestial body" activity.

Teacher Note:

If you have sensitive electronic scales, these could be used to record the actual force exerted by the bottom set of magnets on the top set. These values will not agree with the calculated values of the force. Use this as a "thought exercise" with the students and examine why the discrepancy. If you do not have a set of electronic scales, discuss with the students if they think their calculated values represent the true force being exerted and why or why not.