Ocean Tides
Answer Key
Vocabulary: gravity, high tide, low tide, neap tide, range, spring tide, tides
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
[Note: The purpose of these questions is to activate prior knowledge and get students thinking.
Students are not expected to know the answers to the Prior Knowledge Questions.]
1. A boy builds a sand castle near the ocean. When he returns several hours later, the castle is
gone. What do you think happened?
Answers will vary. The tide came in and washed the castle away.
2. A boat sails into a bay and drops anchor. Several hours later, it is surrounded by mud. What
happened?
Answers will vary. The tide went out, leaving the boat stranded.
Gizmo Warm-up: What is a tide?
In the Gizmo, the pane on the left shows the position of the Earth and Moon. The Sun is far off
in space to the left. The person standing on Earth represents the location of the fisherman
shown at right.
1.
Click Play (
) and observe the ocean depth for several days. What do you notice?
The depth of the water changes over time, going up and down with a steady rhythm.
The rise and fall of water that you see are called tides.
2. Turn on Show value. Click Pause ( ) when the
water is at its highest level, or high tide. What is
the water depth at high tide?
Accept any answer between 5.5 and 5.9 meters.
3. Click Play, and then Pause when the water is at its
lowest level (low tide). What is the water depth at
low tide?
Accept any answer between 2.8 and 3.2 meters.
Activity A:
The Moon and
tides
Get the Gizmo Ready:
 Click Reset (
).
Question: What causes the tides?
1. Observe: Click Play and observe for several days. Try to find a connection between the
tides shown at right and the position of the observer shown at left.
What do you notice? When the tides are high, the observer is pointing toward the Moon or
directly away from it. When the tides are low, the observer is pointing between the Sun and
Moon. [Many students may not have noticed this yet.]
2. Form a hypothesis: What do you think causes tides?
Accept any reasonable answer. Students should notice the position of the Moon during
changes in tides.
3. Observe: Click Play, and Pause the simulation at several consecutive high tides. Each time,
look at where the observer on Earth is pointing. Do you see a pattern? What is it?
The observer is pointing toward the Moon or directly away from it.
4. Observe: Observe several low tides. Where does the observer point during the low tides?
The observer is pointing in a direction 90 degrees away from the Moon. Lines from the
center of the Earth to the moon and to the observer would make a right angle.
5. Extend: Turn on Show tidal bands. The tidal bands show the depth of water at different
places (not to scale). Notice the two bulges that show high tide. Click Fast forward ( ).
A. In what directions do the bulges always point? Toward and away from the Moon.
B. How does the Moon seem to affect the tidal bulges? As the Moon orbits Earth, the
tidal bulges are always lined up with the Moon.
6. Think and discuss: The Moon’s gravity pulls on Earth. How does the Moon’s gravity affect
the oceans closest to the Moon? Farthest away?
The Moon’s gravity pulls on the oceans, creating a bulge that always points at the Moon.
There is also a bulge on the opposite side of Earth. [Students probably won’t be able to
explain this bulge, but they should notice that it is there. The Teacher Guide contains a
fuller explanation of this tidal bulge.]
Activity B:
The Sun and tides
Get the Gizmo Ready:
 Click Reset (
).
 Make sure Show tidal bands is on.
Question: How does the Sun affect tides?
1. Observe: Click Fast forward. Observe the shape of the tidal bands. How does the shape
change as the simulation plays? The shape changes from oblong (oval) to nearly circular.
2. Gather data: Click Reset. Use the Gizmo to fill in the table, recording one high and one low
tide each day. Calculate the range, the difference between high and low tide, for each day.
Answers will vary, but should be close to the values given below.
Day
Depth at high tide
Depth at low tide
0
5.9 meters
2.8 meters
1
5.8 meters
2.9 meters
2
5.7 meters
3.0 meters
3
5.6 meters
3.1 meters
4
5.4 meters
3.3 meters
5
5.3 meters
3.4 meters
6
5.2 meters
3.5 meters
Range
3.1 meters
2.9 meters
2.7 meters
2.5 meters
2.1 meters
1.9 meters
1.7 meters
3. Investigate: Tides with the largest range from high tide to low tide are called spring tides.
Click Reset to observe the positions of the Sun, Earth, and Moon during a spring tide. Then
Fast forward to another spring tide. (Look for the label in the upper right corner.)
What do you notice about the positions of the Sun, Moon, and Earth during spring tides?
During spring tides, the Sun, Moon and Earth are lined up.
4. Investigate: Tides with the smallest range from high tide to low tide are called neap tides.
Click Fast forward, and use the label to help find two periods of neap tides.
What do you notice about the positions of the Sun, Moon, and Earth during neap tides?
Neap tides happen when the Sun, Earth, and Moon form a right triangle. [Or equivalent
answer.]
5. Conclude: How does the Sun affect tides?
When the Sun is lined up with the Moon, they have a combined force, making the tides
stronger. When the Sun is not aligned with the Moon, it competes with the Moon.