Beginning Level Activity
1. Display an image of Adriaen de Vries' Juggling Man. Give students one to two minutes to look at the object closely. Pass out journals and instruct the students to draw the outline of the figure. This will help them to look even closer at the artwork. Introduce the term balance to the class. Ask students to hypothesize about whether the statue is balanced and record the hypothesis next to their drawing.
2. Split students into pairs. One student will take the pose of the figure in the sculpture. The other student should double-check the pose and help correct it, if necessary. He or she will adjust the other's body until it is as close to the Juggling Man's pose as possible. Ask the students in the pose to describe to their partner how it feels to stand on one leg with their arms in the air. Ask students to describe which muscles feel like they are being worked the most in this pose. Which direction do their bodies lean to help them hold the pose? If they lean in the opposite direction, what happens? Students should record their answers in their journals.
3. Introduce the terms push, pull, and force. Ask one student to take the juggler's pose in front of the class. Explain to the class, using the student as an example, how our muscles push and pull by exerting force (using strength) to keep the body from moving in other directions. Put force (using your strength) on the student's hand by pushing on one of the hands that the student is holding up in the air. As you do this, ask the student to use his or her muscles to push back against you while keeping his or her arm in the same place. Ask the student to describe to the class what he or she has to do in order to hold the pose while resisting the force being placed on the hand. Do this exercise again, but this time ask the student not to push back and see what happens. How is the arm affected? How does the pose change? Finally, pull on the student's leg that is in the air. Have the student describe what he or she needs to do to maintain the pose. Students should record their observations in their journal.
4. Display the side view of the Juggling Man showing the S-curve of the figure's back. Tell student that the artist has arranged the body into an S-curve to combat gravity by balancing force to keep the figure upright. Share the background information about the artwork with the class. Based on their experimentation with poses and findings from the discussion, students should be able to determine which parts of the statue are exerting force to balance a push or a pull.
Intermediate Level Activity
5. Divide students into groups of three. Explain that students will explore how objects of different weights can affect balance. They will predict how a balanced pose will need to change in order to accommodate objects of differing weights. One student will take the pose of the Juggling Man. Another student will arrange the body to make sure it is in the correct position. One student will then place objects of varying weight in the hands of the student holding the pose. The third student will document the objects used and the effects of adding the weight on the Force and Balance Observations student handout. For each object, do the experiment twice: The first time, place an object in the posing student's hands and instruct the posing student not to resist. Record what happens. What direction does the pose 'fall'? Students should then predict what the posing student has to do to adjust the balance. Then, place the object in the posing student's hand a second time, but this time the posing student should adjust their pose to stay balanced. Record the adjustment in force that was required to keep the balance. Does this reaction match what students predicted? Encourage groups to apply forces to other areas of the body in addition to the hands, experimenting with force and reaction and recording results.
6. Share with the class the way in which Adriaen de Vries created his bronze by watching the video Casting Bronzes. Explain that bronze has a unique property called tensile strength due to its density. By taking X-rays, conservators found that some areas of the statue are thicker than others, but still remain balanced, such as the solid metal hands resting on hollow arms. Both the tensile strength of bronze and the thicker areas in the sculpture counteract the force of gravity, helping to keep the sculpture balanced and preventing it from breaking from the weight of the metal. The thicker areas of metal are similar to the muscles in your arm, which could resist the weight of the objects you were holding when you increased your muscle strength.
Advanced Level Activity
7. Discuss Newton's Third Law of Gravity with the class: "For every action, there is an equal and opposite reaction." Ask students to describe what this statement means. After students discuss, explain that there are always two forces acting in opposition that keep an object stable. The two forces on the object are equal—the direction of one force on the object is moving in the opposite direction from the other. Forces always come in pairs of equal and opposite—action and reaction. For instance, if you drop a ball, gravity pulls the object down towards the earth. When it hits the ground, the force of the earth pushes it back up to create the bounce.
8. Introduce the terms centripetal force and centrifugal force. Explain that de Vries depicted a man juggling. Juggling is an example of this action-reaction pair. If the sculpture were to come to life, the man's hands would push on the plates to create a centrifugal force that would send the plates along circular paths in the air. When a juggler stops moving his arms, the objects being juggled continue to move. But without the initial opposing force of the juggler's arms, the objects move in a straight path towards the floor as described by Newton's First Law of Gravity. The juggler's arms create a force that opposes the force of gravity—centripetal force—to keep the object being juggled in the air in a circular path. When juggling, centripetal force is created when the object makes contact with the juggler's hand which in turn sends the ball in a new direction and keeps it from falling to the ground.
9. Look at the artwork again. Ask the students to describe how the man is holding the plates in each hand. Ask them to describe the shape of his arms. Students should notice that his arms are curved and that the plates are barely touching his hands, showing that the artist is capturing a snapshot of the man juggling the plates. Students should identify the action-reaction pairs in the artwork. If students have trouble understanding the centripetal/centrifugal concept, the animations on the Physics Classroom Web site could be helpful.
10. Have students sculpt their own balanced figure of an athlete in motion. Gather various pictures of athletes in dynamic poses, such as a person swinging a bat, a person kicking a ball, or a dancer doing a turn. Images can be found in magazines or newspapers. Students may also bring in images from home. Students will choose one image from which to create a sculpture. Give each student modeling clay so that he or she can sculpt a human figure in balance. Students should give special attention to the direction of body parts and how they compensate for the force being applied on the body. Students may use plastic utensils and Popsicle sticks to sculpt the clay. They may also make some areas thinner and others more solid to accommodate the force of weight and gravity. Once completed, sculptures should be freestanding and balanced.
11. Look closely at the sculptures. Which body parts are creating a force? Where do students see the reaction to the initial force? Students may work with a partner to identify where the artists have depicted action-reaction pairs. Students may take the pose of each sculpture and mimic the action depicted. As they move, they should play close attention to where they feel a push or a pull and the reaction that happens to keep them in balance.
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| Juggling Man, Adriaen de Vries, about 1610–1615 |
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