TIDES notes page 1
TIDES
Earth/Space Science
Note: It is probably a good idea for students to work through phases of the moon before they try to understand the tides. If they don’t understand the motion of the Earth and Moon then they will have trouble understanding the tides.
1. Ask students what the tides are and why there are tides. Collect answers. Ask students why there are tides. Collect answers (“the moon”, “gravity”, etc.).
2. Ask students to imagine that the Earth was a ball of rock covered by water, as in the picture on the left below. Ask them to draw a picture of what happens to this picture when it is acted on by the gravitational pull of the moon. Collect answers, draw some of the diagrams large enough for all students to see, and have students discuss the answers.
A likely answer/ Another likely answer
3. Ask the students how many high tides there are in a 24-hour period. Many will not know. Produce tide tables or simply tell them that there are two high tides and two low tides every 24 hours. Ask them how many high tides there would be in 24 hours if the diagrams above were correct. Ask them to create a new diagram that accounts for two high tides in 24 hours. Remind them that the high tides are always just about 12 hours apart.
A diagram that accounts for two high tides and two low tides per day.
Your students may have seen this diagram in textbooks.
4. Your students should determine that a diagram similar to the one above is required for two high tides spaced 12 hours apart. Once they have understood this, repeat the original question: “What causes the tides?” Especially, “What causes the high tide on the side of the Earth away from the moon?”
Misconception warning: If your students do not understand the motions of the Earth and Moon about the Sun, some will insist that the diagram can be explained by the Moon pulling on one side and the Sun pulling on the other. That would not account for two tides consistently spaced 12 hours apart (since the Sun and Moon are only on opposite sides of the Earth during full moons). This is why an understanding of phases of the moon would be useful before trying to understand the tides. If your students understand phases of the Moon you can give them the added puzzle: BOTH high tides are higher when the Sun and Moon are on the SAME SIDE of the Earth (new moon). Why is that?
5. Give students a model of the Earth and the Moon in which both parts are free to move (misconceptions are reinforced if the Earth is nailed down). A couple of lengths of rubber hose joined in loops work well. Tie them with elastic cord or rubber bands. Have the students place the hoses on the table and stretch the elastic cord. Aha! The Earth forms an ellipse! Remind the students that there are no hands holding the Earth and Moon apart so they should let go of their models. Oh oh! The Earth and Moon just collided! Ask your students why this doesn’t happen in space. Somebody will suggest orbital motion (which is correct).
6. Give students asymmetric objects to spin on the table. Students can make their own by taking rulers and taping weights at each end with one weight much heavier than the other. Ready-made examples are screwdrivers, hammers, and heavy-handled scissors. Ask your students to describe the motion when the objects spin. Notice that the light end is not the only end that is moving! BOTH ends are in orbit around the center of mass.
7. Ask students to hold heavy objects (their backpacks are great) in front of their chests at arm’s length. Now spin so that the object (your moon) is in orbit around you (the Earth). As other students watch they will see that students can’t do this without leaning back. Ask them what this tells them about the motion of the Earth and the Moon. (The Earth and Moon are BOTH in orbit around the center of the mass of the system – and that is not in the center of the Earth.)
8. Now ask them to toss their Earth-Moon models into the air “in orbit”. This takes practice and they will need to take turns because the one doing the throwing can’t see what happens. The Earth again is stretched into an ellipse and it wobbles as it orbits a point that is a little bit off-center. This wobble creates “centrifugal force” which creates the “other” high tide in our oceans.
Note: as with many things in science, there is not just one “right answer” to the question of what causes the “other” tide. Some students may suggest that the Moon’s gravity is not as strong there because it is farther from the moon. This is true but it only accounts for a part of the tides we see. Others will suggest a large wave. This is true, too, but it does not answer the question of where that wave comes from. Students may ask lots of hard questions. Encourage them to investigate on their own.