Teacher's Pages
Objective:
Students will discover, through accurate measurement, that the length of a shadow is shortest around noon when the sun is at its highest in the sky.
Background:
Every day the sun appears to cross the sky from east to west because of the earth's rotation. For thousands of years people have noticed that shadow length changes during the day. People have also noticed that the length of shadows
increases after the summer solstice and decreases after the winter solstice. This change happens because the sun's position in the sky changes with the seasons. The sun is lowest in the sky in winter and highest in summer. On a daily basis, the length of shadows decreases from a maximum at sunrise to a
minimum at noon. From noon to sunset, the shadows become progressively longer again. A common misconception is that at noon the sun is directly overhead and there are no shadows. A little investigation will dispel that notion. In fact, the sun is never directly overhead anywhere at anytime in the continental United States.
In winter months the sun is low in the southern sky and shadows cast will be longer than those made in the summer. After the winter solstice, the sun slowly gets higher in the southern sky and shadows cast at noon slowly get shorter until the summer solstice in June.
When designing passive solar homes in the United States, builders take into account the sun's changing position in the sky. In winter the sun is low in the southern sky and south facing windows can let the sunshine in and contribute to warming the house. During summer when the sun is high in the sky, overhangs or awnings over the windows block the suns rays, preventing solar energy from warming the house too much.
Materials:
ruler, clay, yardstick, protractor
Teaching Suggestions:
1. Encourage your students to make the ruler as vertical as possible. Allow them to use a protractor if they wish.
2. Have the students make the observations so that noon (standard time) measurements are included. It would be good if you could start the activity early in the morning and continue until close to the end of the school day.
3. If you have students measure to the nearest 1/16 of an inch instead of using metric units, they will get practice adding and subtracting fractions with uncommon denominators.
4. Encourage students to look for patterns in the shadow lengths, i.e. they grow shorter as noon approaches and grow longer after noon.
Extended Activities:
Have students pretend they live on a planet that has two suns. Write a story about how shadows change on such a planet.
Problem:
How does the length of a shadow change during the day?
Materials:
12 inch ruler
Clay
Yardstick
Follow This Procedure:
1. Find a sunny, flat area around your school. Use pieces of clay to support a 12-inch ruler straight up and down.
2. Measure the length of the ruler's shadow with a yardstick. Record the length of the shadow and the time of day in a table.
3. Predict when the shadow will be shortest.
4. Measure the shadow's length, in inches, every 30 minutes for three hours. Record the time and shadow length after each measurement.
5. Estimate and then calculate the change in the shadow's length since the last measurement. Look for a pattern.
Think About It:
1. How does the shadow length change from morning to afternoon?
2. When was the shadow shortest?
3. What is the difference in length between the longest and shortest shadow you measured?
4. Why do you think shadow length changes?
Teacher's Pages
Objective:
Students will measure and record wind speed by observing the wind's effect on a suspended Ping-Pong ball.
Background:
Wind is air in motion. Our air is composed of molecules and atoms of gases, mainly nitrogen, oxygen and argon. When these small particles move and strike
an object, a force is exerted. Students know that strong winds can knock down power lines, break branches from trees and remove roofs from houses. Wind speed increases with increased distance above the ground. This is because objects on the earth's surface such as trees and buildings exert friction on moving air particles and actually slow the wind.
In this activity a table tennis ball is suspended vertically and exposed to the wind. Wind displaces the ball an amount dependent on the wind speed. Students observe the angle of deflection and use a table to relate the angle to wind speed.
Materials:
protractor, ruler, clear tape, 25 cm red thread, table tennis ball
Teaching Suggestions:
1. Large protractors with a hole at the measuring center work very well for this activity. It is important to suspend the string from the protractor's measuring center.
2. Winds swirl around buildings and trees, changing speed and direction. Students can investigate this phenomenon if you wish. However students should measure actual wind speed in an open area.
Extended Activities:
Have students convert the wind speeds from kilometers per hour to miles per hour, a more common measurement for them. One kilometer per hour is equivalent to 0.61 miles per hour. For example, 10 kilometers per hour = 6.1 miles per hour.
Problem:
How can you build a tool for measuring wind speed, and measure wind speed?
Materials:
protractor, ruler, clear tape, 25 cm red thread, table tennis ball
Follow This Procedure:
1. Use a table to record your observations.
2. Tape the protractor to the ruler as shown in the picture.
3. Tape one end of the thread to the table tennis ball. Tape the other end of the thread to the protractor as shown. The thread should hang down the middle of the protractor.
4. Take your measuring tool outside. Hold it still using the ruler as a handle. Point the ruler into the wind. Be sure the ruler is level with the ground.
5. When the wind blows the ball, it will move the thread on the protractor. Record the highest number on the protractor that the thread reaches.
6. Find the wind speed number in kilometers per hour under this number in the chart below. Record the wind speed in your table.
7. Measure and record the wind speed 3 times during the day.
Number on protractor and corresponding wind speeds:
90: 0 km/hr; 95: 9; 100: 13; 105: 16; 110: 19; 115: 21; 120: 24; 125: 26; 130: 29; 135: 31; 140: 34; 145: 37; 150: 41; 155: 46; 160: 52
Think About It:
1. What was the highest wind speed you measured?
2. How can you use a number on a protractor to find wind speed?
3. For a windmill to make electricity, wind speed must be at least 13 kilometers per hour. Could windmills be used where you live? Explain your answer.
Teacher's Pages
Objective:
Students will use area of polygon formulas to find that the area of a wind vane's tail must be larger than the area of the pointer in order for the wind vane to work properly.
Background:
A wind vane is used to determine the direction of the wind. Wind vanes are usually made with a large tail to catch slight breezes. They turn to point into the direction the wind is coming from. The directional markings on a vane must
be aligned with north, south, east, and west using a compass. A wind is named for the direction it is coming from. For example, a north wind is blowing from the north. A west wind comes from the west and blows towards the east.
Materials:
pencil with eraser, paper or polystyrene cup, straw, straight pin, scissors, 3" x 5" cards, ruler, glue, and a directional compass
Teaching Suggestions:
1. Make sure you remind students to be careful with the straight pins. Remind them also not to jab each other with the pins.
2. Encourage students to use regular polygons for the shapes of their tails and pointers to simplify finding areas. They can use shapes that are combinations of regular polygons, but these will have to be divided off in order to calculate area.
3. Some helpful area formulas:
Rectangle: A = l x w
Triangle: A = 1/2bh
Parallelogram: A = bh
4. If students are not familiar with these formulas, have them estimate the area by placing the shapes on graph paper that has squares of a known area. Then the students can count the squares to find the total area.
5. You might wish to place a fan in your classroom for students to test their wind vanes. The fan's blades will provide a steady air flow.
Extended Activities:
1. Take the class outside and allow them to determine wind direction.
2. Have students research the history of wind vanes. If you know of a local collector of wind vanes, invite them to visit your classroom.
Problem:
How can you make a working wind vane?
Materials:
Paper or polystyrene cup, Pencil, Straight pins, Scissors, 3 x 5 inch index cards
Follow This Procedure:
1. With a pencil, make a hole in the bottom of a paper cup. Push the pencil through the hole as shown in the picture. Cut slits about 1 cm long in each end of a straw. Push a straight pin through the center of the straw and into the eraser of the pencil so that the slits in the straw are vertical.
2. Predict whether the wind vane will work better when the tail is larger than the pointer or when the tail is smaller than the pointer.
3. Cut the pointer and the tail for your vane from 3 x 5 inch cards. Estimate and then calculate the area of each. Insert the pointer and tail into the slits in the straw and glue them in place.
4. Test your wind vane to see how it works. Experiment with tails and pointers of different sizes. Calculate the area of each. Record your results in a table.
Think About It:
1. On what basis could you rate the performance of a wind vane?
2. How do your results compare to your prediction?
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