Teacher Note: In this lesson, subtraction will be demonstrated using a pan balance. If you have only a balance with hanging weights, please modify the directions accordingly. You may wish to use heavy weights such as sinkers for this lesson so that the balance will clearly register the differences in weight. The pieces should be identical so that groups of the same size weigh the same amount.
To review with the students how a pan balance operates, display one and call on a volunteer to "be" a pan balance. Ask the child to hold out his or her arms, then place a container in each hand. Tell the child to imagine that the
container on the left side is very heavy, and to act out what the balance will look like. (You might also place a heavy object in the hand that represents that left side of the balance.) Then ask him or her to imagine the left side is much lighter than the right side and act that out. Tell students they will “act out” the behavior of a pan balance. Then ask the whole class to stand, stretch their arms out and show how a pan balance works. [You may wish to suggest which side of the pan balance is heavier, lighter.]
Now ask a volunteer to place nine weights in the left pan of the balance and then put four weights on the other side. Ask: "How many more weights are on the left side than on the right side? What will we need to add to the left side so the pan balance is level?" Accept and model all the students' responses. When the response "five" is given, record this on the board using an equation [9 = 4 + 5] that models what was done with the weights to make the pan balance level. (Note that although the number sentence is an addition sentence, subtraction is used to find the missing addend.) Continue with other examples until the
students are comfortable with the process. (Because this is an example of a missing addend problem, some students may have difficulty completing the equation correctly.) To continue the investigation, call on volunteers to pose
problems and model them on the pan balance.
Now give each pair of students some crackers and a copy of the Pan Balance activity sheet.
Tell students they will take turns choosing two numbers and acting out how a balance would look if there were that many crackers on the left and
right sides. Ask students to determine how many more crackers would be needed to balance the pan and record that information on the recording sheet. Then ask them to write the number sentence that they modeled. Invite the students to select one of the resulting equations to illustrate by drawing a pan balance that models the equation.
