Balancing Act

Grades: K-4

By Marlene Davis, Margaret Kruse, Janet Herman,Carol Schultz, and Cindy Rodenbuagh

Principal Investigators: Dr. Pat Cowings and Marilyn Vasques

Overview

In this activity, student scientists explore balance. They identify forces that enable them to balance an object on their hand, then construct a balance and use it to compare the mass of objects. They learn the terms lever and fulcrum, and discover that a mass on one side of the fulcrum can balance with another mass on the other side of the fulcrum.

Maneuvering a ship in space or in water on Earth applies the concepts of force and balance. When the jets on the Space Shuttle are fired they apply force to the space ship. The force of the jets must be carefully balanced so the ship will not spin out of control.

Key Questions

• What does it mean to say that something is balanced?
• How can you tell if something is balanced?
• What forces are involved in balancing a lever on a fulcrum?

Time frame

Experimenting with balancing rulers: 20 minutes

Using a balance to compare the mass of objects: 20 minutes

Wrap-up: 30 minutes

Materials:

For each student:

• lever: ruler, yardstick, or any flat stick or board
• fulcrum: small block, small box, can, or paper roll tube, etc.
• Space Shuttle Orbiter model and string

Note: If the Space Shuttle Orbiter model does not print well , notify the STELLAR Program and request hardcopy to be sent to you by mail.

For each group of 3-5 students:

• miscellaneous objects of the same size and weight: coins, blocks, paper clips, etc.

Getting Ready

1. Get the levers and fulcrums (one each per student).

2. Have the miscellaneous objects in boxes ready to distribute for group use.

3. For very young students, have Space Shuttle Orbiter models made in advance by older students, parents, or other "free" labor.

Classroom Activity

1. What is balance? Hold up a ruler on your palm. Ask, "Is this ruler balanced?" (Accept answers.) "How do you know that this ruler is balanced?" (Accept answers.) Explain that the word "balance" can mean many things. We say the ruler is balanced because if it were not balanced it would be falling. Drop the ruler. Once it is resting on the floor, ask, "Is the ruler balanced now?" (Accept answers.) Explain that when the ruler was not balanced on your hand, it moved to the floor where it is now balanced.

2. Balancing the pull of gravity. Ask, "Why did the ruler leave my hand?" (Accept answers.) Explain that when the ruler falls, it's almost like some invisible person reaches up and pulls the ruler down. That invisible pull is called gravity. As long as a hand pushes up on the ruler just as much as gravity pulls down on the ruler, the ruler does not move-it is balanced. If a hand is not there, the ruler falls because there is nothing to push upwards and balance the pull of gravity. Drop the ruler again. Ask, "What is pushing up on the ruler and balancing the pull of gravity now?" (The floor.) [Note: for older students, you may choose to use the word "force" in some places where we have used the word "pull."]

3. Why do things move up or down? (Optional) Ask, "What happens if the pushing [force] of my hand upwards on the ruler is stronger than the pull [force] of gravity pulling the ruler to the floor?" (Accept answers.) Explain, "When I lift the ruler from the floor, the ruler rises because the pushing of my hand on the ruler is stronger than the force of gravity pulling the ruler to the floor. What would happen if we somehow removed the floor from under the ruler?" (Accept answers.)

4. Playing with balance. Hand out rulers and allow students time to experiment with lifting, holding, balancing and dropping their rulers. Check for understanding by asking them to describe the forces acting on the ruler as it is lifted, balanced, and falling.

5. Balance on a finger. Ask the class, "Do you think I can balance the ruler using just my finger?" Demonstrate using your finger to balance the ruler. Challenge the students to balance their rulers on their own fingers. After a while, have students watch you balance the ruler on your finger again and explain that we know it is balanced because it is not falling. We can also say that each side of the ruler balances the other side. We know when the two sides do not balance each other because the ruler tips. Demonstrate tipping.

6. Which side has more material? Explain that when the ruler is "off balance" and tips, more of the ruler is on one side of your finger than the other side. Ask, "Does gravity pull more on the side with more ruler or on the side with less ruler?" Let students experiment to see if their answers are correct-tipping, balancing and dropping the rulers. Reinforce the idea that the ruler tips to the side where there is more ruler, or more material. Since there is more of the ruler on one side of your finger, there is more material there for gravity to pull. So the side with more material tips towards the Earth, and the side with less material tips toward the sky.

7. Making a balance. Give a block (or whatever fulcrum you have) to each student. Challenge them to balance the ruler on the block. Review and check for understanding, "When the ruler is balanced on the block, does gravity pull down on the ruler more than the block pushes upwards?" (No.) "Does gravity pull on the ruler less than the block pushes upwards?" (No. The pull is the same.) "When the ruler is in balance does gravity pull more on one side of the ruler than the other side?" (No, it pulls equally on both sides.) Reinforce the idea that gravity pulls equally on both sides of the ruler because there is the same amount of material [or mass] on each side of the block."

8. Parts of a balance. Tell the class, "You just made something called a balance. The balance is made of two parts: a lever, and a fulcrum." Point to the parts as you introduce the terms and have students pronounce the words. Explain that the lever is the ruler, or any stiff bar which rests on the fulcrum, while the fulcrum is the block, or any object, which supports the lever. With a balance, you can compare the amount of material [mass] in objects.

Optional:: You may want to explain that a lever is often used for lifting objects. Demonstrate by placing an object on one end of your balance and pressing on the other end to lift the object. A see-saw is also a lever.

9. Using the balance. Place an object at one end of the ruler. Ask, "Are the two sides of the lever balanced?" (No.) "Why isn't the lever balanced now?" (There is more material [mass] on one side of the balance than the other.) Ask, "What can I do to balance the lever on the fulcrum?" (Accept answers.) Challenge the students to add mass to one side of their lever and then to balance the lever by adding mass to the other side.

10. Change where the fulcrum is. (Optional) Students may also adjust the position of the lever on the fulcrum in order to make it balance. You might point out that the objects and the ruler have mass. If an object is placed on one side of the lever, the lever or fulcrum can be moved so the mass of the lever itself balances the object. Another factor that your students may discover while playing with their balances is that the balance condition depends not only on the mass, but also on how far the mass is from the fulcrum. There is a mathematical relationship to describe this relationship, but this detail is beyond the scope of this activity (see Teacher Background if you're curious).

Wrap-up Session

1. Controlling a space ship. Imagine you are the captain of a long space ship with jets in various places as shown in the picture.

a. Which jets would you fire if you wanted the ship to move straight up? Straight down? To turn? Go forward? Is there a way to make this ship go backwards? How would you do it? [Color-code or number the jets, to facilitate discussion.]

b. Ask the students to predict how the ship will move as each jet or combination of jets is fired. How is moving the space ship like balancing a ruler on a fulcrum?

2. Design a spaceship. Have students design their own space station or submarine with jets in various places. Students can role play making their ships move by giving commands to fire the jets.

3. Shuttle Orbiter model. Have students build the Space Shuttle Orbiter model included with this activity (template on page after materials list). For younger students not able to do this, arrange to have the models pre-made either by older students, parent volunteers, etc. Point out to students the small rockets numbered 1-16 on the model. These rockets are part of the "Reaction Control System" which control how the Orbiter moves in space. Note the box that has the key indicating the direction of thrust for each rocket.

Have students hang their Orbiter models by string (about 1 meter long). Students may work in small groups for the following:

a. Explain, "When rocket number five is fired, it creates a push [thrust] on the ship at that spot. Pretend that rocket is firing by pushing with the tip of your finger on the spot marked five. How did the ship move when you 'fired' rocket number five?" (It turned.) "Where would you have to push to make the ship move sideways without causing it to turn?" Let students push the ship in different places and experiment to come up with an answer. Explain that rocket number ten also pushes to the side. Have students "fire" rocket numbers 5 and 10 simultaneously by pushing those spots. Have them do similar experiments with rockets numbered 8 and 14 which also thrust outward from the side of the ship.

b. Explain that rockets 6 and 7 thrust downward. Students can "fire" those rockets by pushing upward under the nose of the ship. Ask, "How did the ship move when you 'fired' rockets 6 and 7?" (The nose flipped upward) "Where would you have to push to make the ship move upward without tipping?" Allow time for experimentation. After a while, suggest that they "fire" rockets 11 and 15 (that thrust downward) by pushing upward with a finger under the tail of the ship. They can fire 6, 7, 11 and 15 by pushing upward under the nose and tail of the ship simultaneously. How does the ship move if rockets 6 and 11 only are fired? (push upward from under one side of instead of directly under the nose or tail of the Orbiter model).

c. Have students "fire" each of the numbered rockets singly and in various combinations. Tell the students, "Imagine that you are the commander of the Space Shuttle Orbiter. Give the commands to fire the rockets necessary to make the Orbiter move in the manner you desire."

More Activity Ideas:

1. Play with different levers and fulcrums. What happens when the lever rests on a smaller fulcrum? Is the balance is more sensitive?

2. Compare the mass of various objects as in the Spring Scales activity, only using the balance instead.

3. Use commercially made balances.

4. Use coins, blocks, washers, etc. as units of measure. Compare the mass of different objects by comparing the number of washers necessary to balance the objects. Make a chart listing each object and how many washers were necessary to tip the balance. Students can report their findings by saying, "This object has a mass greater than _x_ washers but less than _y_ washers."

5. Use a see-saw or other large set-up to do a group demonstration comparing mass of larger objects, including students themselves.

6. Using a finger as a fulcrum compare the balance point of objects with odd shapes to those which are symmetrical.

Background for Teachers:

Prerequisites:

Students must be able to use their hands. They must be able to lift, hold and balance a ruler using their fingers.

Vocabulary:

• lever-a rigid bar that can be placed on a fulcrum and have force applied to it at various points. A lever may be used to lift or pry objects in addition to being used in making a balance.
• fulcrum-The support on which a lever turns. By placing the fulcrum at different places, the lever is able to lift, balance, and move objects.
• balance-1. an instrument for weighing. This is usually a beam or lever supported exactly in the middle and having two pans of equal weight at each end. 2. state of equilibrium, steadiness; an object that is not falling is said to be balanced.
• mass-a quality of an object dependent on the amount of matter it contains.
• gravity-the force which pulls every object on the Earth towards the center of the Earth.
• force-1. a push or pull. 2. An influence which produces motion or change of motion.

Skills:

• precision alignment of objects on a lever and a lever on a fulcrum
• identifying forces involved in a system at equilibrium.

Concepts:

Balance is a term that applies to many situations. An object is said to be balanced if it is not falling. An instrument called a balance can be constructed by balancing a lever on a fulcrum. This instrument is used to compare the mass of objects which are placed at opposite ends of the lever.

Additional Background information:

As promised, here is the exact mathematical relationship between mass and distance from fulcrum in a balance: When balance is achieved, the product of mass and distance on one side of the fulcrum is equal to the product of mass and distance on the other side of the fulcrum. This very simple relationship is complicated by the fact that the lever actually has its own mass distributed along its entire length.

The concept of balance is applies to many everyday experiences. We can look at things in terms of systems including objects and the forces they exert on each other. Equilibrium exists when the system does not appear to be changing, such as system of balanced objects. Imagine a system consisting of a ruler on a block with two identical objects next to the block, all resting on the floor. This is a system in equilibrium. If you place one of the objects on one end of the ruler, the ruler tips. While the ruler is tipping, the system is no longer at equilibrium. When the ruler stops tipping, the system has regained equilibrium. A system remains in equilibrium unless an external force acts on the system. The ruler stays tipped until another force acts on it. When you put the other object on the other side of the ruler, the system moves to regain a state of equilibrium. The term balance, of course, has many other common uses. For example, we say a person is well balanced if they have strengths in several areas.

Dynamic Equilibrium and Orbits

Some systems may be in dynamic equilibrium., often found in natural systems when objects in the system are moving, yet overall, the system is not changing. For example, in the system which includes the Earth and the Sun, the Earth is moving around the Sun. But it is in equilibrium because its orbit does not change. The Earth stays in it's orbit because the force of the Sun's gravity pulling the Earth towards the center of the Sun balances the natural tendency of the Earth to remain moving in a straight line due to Earth's own momentum. What force is balancing the Sun's gravity in this system? People sometimes refer to a "centrifugal force" that counterbalances the Sun's gravity. But there is no real force as such-"centrifugal force" is really just the natural tendency of the Earth to remain moving in a straight line due to its momentum.

This activity was edited by: Gregory Steerman (Lawrence Hall of Science)

Keywords: Balance, Lever, Fulcrum, Force, Gravity