OCEANOGRAPHY: WHAT CAUSES TIDES?

Title: What Causes The Tides?

(Gravitational Pull, Centrifugal Force)

Grade Level(s): 6-8

Introduction: Ocean tides have perplexed people throughout history. One ancient theory said that a water god would swallow water and release it, thus causing the high and low tides. These changes in water level have been explained today, of course, but remain a mystery to many. A high tide (or flood tide) is when the water is high and the beach is covered. Low tide (or ebb tide) is when the water recedes.

Although tides are caused by the gravitational force of the moon, many other factors come into play. A hands-on exercise, described later, physically shows the center of gravitation between two objects and allows students to better understand that the real moon and earth share this same phenomenon. In the activity, the center of gravitation is not exactly between the two objects but showed towards the larger one. The center of gravitation for the earth and the moon is actually within the earth itself. As the students rotate their model around the center of gravitation, they will notice that the spinning objects lean outward. This is called centrifugal force. This centrifugal force causes the water on the opposite side of the earth to bulge and rise. The moon also attracts the water on the side nearest to it and this creates a second bulge.

Learner Objectives:

• The student will be able to create a model demonstrating ocean tides.
  
  
• The student will write a short paragraph describing what causes ocean tides.

Florida Sunshine State Standards: Math: MA.E.1.3.1. / MA.A.3.3.3. / MA.E.1.3.3. / Science: SC.C.2.3.7.

Competency Based Curriculum: Math: M/J-1 - V-2-A, II-1-A, M/J-3 - VI-7-A; Science: M/J-3 - I-8-A

Materials:

meter stick
tether ball (or ball of similar size)
tennis ball
heavy string

Activity Procedures:

1. Discuss ocean tides with students, eliciting prior knowledge and discussing myths.
   
   
2. Discuss causes of tides and review the following vocabulary:
   
   • tides
   • center of gravitation
   • centrifugal force
   • tidal bulge
   • flood time
   • ebb tides
     
     
3. Have students create a model using string, a meter stick, and two balls. Begin by fastening the two balls to opposite ends of the meter stick using the string. Then rest the meter stick on your finger until the two balls on either end of it are perfectly balanced. The point on which your finger is rested (nearest to the larger ball) is the center of mass between the two objects. Tie a piece of string to this point. If you were to cut the meter stick in half at this point, both sides (stick and ball) would weigh the same.
   
   
4. Next , grasp the stick by the string and gently rotate the objects simulating the rotation of the earth and moon around their center of gravity. (Note: The actual center of gravity would be inside the earth.)

Student Assessment:

Have students write a paragraph explaining the causes of ocean tides.

Activity Extensions:

Ask students why it appears that the moon is revolving around the earth? (They are revolving around their center of gravitation and this is actually located within the earth’s crust.)

Home Learning Activity:

Ocean tides are not exactly 12 hours apart. Tides are 1/2 of a day + 1/2 times 50 minutes apart. Check your local newspaper for current tidal information. Try to predict the high and low tides for the next week. Present your data on a chart or graph.

Vocabulary: tides, gravitation, centrifugal force

References/Related Links:

http://www.cln.org/themes/earlynavigation.html
http://www.odysseyexpeditions.org/oceanography.html
www.aquanet.com