BioEd Online

Teacher Background

Gravity places a heavy load on the human body. But good direction from the nervous system and coordination among muscles in the back, legs, ankles and feet enable us to counteract the downward pull of gravity and remain balanced and upright.

To balance itself, the body makes continual tiny corrections to maintain its center of gravity over the feet, an imaginary point within the body at which there is balance and from where the weight on all sides is equal. Fortunately, the muscle adjustments necessary to maintain balance and posture are directed automatically by the nervous system.

This activity will help students begin understanding how one’s “sense of balance” affects his/her ability to do physical activities.

Objectives and Standards

Texas Essential Knowledge and Skills (TEKS) Objectives

Science

3.2.A-F; 4.2.A-F; 5.2.A-F

• Student uses scientific inquiry methods during laboratory and outdoor investigations.

3.4.A-B; 4.4.A-B; 5.4.A-B

• Students know how to use a variety of tools, materials, equipment, and models to conduct science inquiry.

3.4.A-B; 4.4.A-B; 5.4.A-B

• Students investigate the effects of gravity

Health

3.1.A; 4.1.F; 5.1.E

• Students will recognize and explain ways to enhance and maintain health and recognize and perform behaviors that reduce health risks throughout their lifespan.

Materials and Setup

Materials per Group of Students

• Light-weight chair or stool

• Copy of the “Balancing You” student sheet

Procedure and Extensions

Engage

1. Ask students, Do you usually fall over when you’re walking, riding a bicycle or standing on a bus? Why or why not? Encourage students to think about how the body coordinates balance. Ask, Do you need muscles to keep your balance? Would your skeletal system alone be able to keep you upright?

2. Explain that students will investigate balance and stability while learning how living things use muscles and body position to maintain balance.

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Explore

1. Ask students what our “center of gravity” is, and if it ever changes. Ask, How do you keep yourself from falling when you trip? How do you maintain your balance when you’re standing in a moving train or bus? Tell students that they will be exploring their own centers of gravity in two different ways.

2. First, have students in each group take turns standing up from a seated position in their chairs, and then record the results on their data sheets. Ask, How easy was it to stand up? (very easy)

3. Have students try again to stand up from a seated position in a chair. This time, however, have them do so without leaning forward. Once again, students should record their results.

4. Next, instruct one student to stand with feet shoulder-width apart. Have a second student place a lightweight chair 15 cm in front of the first chair 15 cm in front of the first student. Instruct the first student to try to pick up the chair and then record his/her results. Then have students switch roles and repeat the process.

5. Tell students to move to the periphery of the room and place their heels, hips, back and shoulders against the wall, and keep their feet flat on the floor. Have students take turns attempting to pick up the chair. They should record their results.

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Explain

1. Discuss students’ results, and ask the class to identify the differences between the two trials of each experiment. Ask, Why do you think it was not possible to stand up when you didn’t move your shoulders? Why was it impossible to pick up the chair while you were standing against the wall? Help students understand that in both cases, needed changes in balance could not be made because their body movements were limited.

2. Discuss gravity again. Ask, Does gravity affect people? Do people have a center of gravity? Have you ever observed changes in a person’s center of gravity?

3. Have students think about where their centers of gravity are when they are sitting in chairs, and how things change when they begin to stand up. As their weight shifts from their seats to their feet, their centers of gravity must change also.

4. Have students consider where their centers of gravity are when they lift a chair. The chair adds weight to the body, which must then compensate for that weight by moving the center of gravity.

5. The body changes the center of gravity and achieves balance by moving the hips backward. This is why students were unable to pick up a chair when their backs were against a wall.

6. Have students repeat these two experiments, this time paying close attention to their partners’ body movements.

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Elaborate

1. The body and muscles constantly adjust to compensate for the pull of gravity. Some of the adjustments are large, like those made when we pick up a chair, but many are very subtle, almost unnoticeable.

2. Have students work in pairs. Each student should observe the movements of his or her partner as he/she performs certain tasks, such as moving from a two-footed stance to standing on one foot, walking heel-to-toe, squatting, or standing on tip-toes.

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Evaluate

1. Tell students to imagine a person standing in the aisle of a subway or bus, with nothing to hold on to.

2. Have student groups discuss and describe what would happen to the passenger when the vehicle begins to move, comes to a stop, or turns a right-hand corner. Ask, Without finding something to hold on to, how could the person keep from falling down?

3. Students should realize that the passenger could spread his/her feet out to keep from falling over. Ask, What does this movement accomplish? (It widens the person’s “base” and adjusts his/her center of gravity.)

Handouts and Downloads