Balancing Mobiles

Essential Question
How does a lever work? What are the classes of levers?

Objectives

 * Explain how balance and torque work.
 * Distinguish the difference between the three classes of levers.
 * Build a mobile that models how balance functions.

Introduction & Student Background
Almost anyone will recognize a simple lever like a teeter totter or a can opener. This activity discusses how to consistently describe levers as one of the three classes. The exercise uses class one levers and borders on a hands-on math lesson. It can be used to show the commutative principle, ab=ba. It focuses on how a lever works and breaks down the classes of lever. It also uses simple wooden dowels

Vocabulary

 * lever
 * balance
 * effort
 * torque
 * mass
 * fulcrum
 * resistance
 * mechanical advantage
 * length
 * pivot
 * lever arm

Materials

 * Beaded chain
 * 25 1/2 inch cut watshers
 * 4 1/4 inch wooden dowels, one foot long
 * 14 small binder clips to attach the washers, chains, and rods
 * 4 chain connectors (binder clip chained to a binder clip)
 * 5 weights (5 washers chained to a binder clip)
 * 1 spare binder clip for levers
 * 1 500 gram spring scale
 * 1 250 gram spring scale

Building the pieces (PREP)

 * 1) Cut 4-5 inch lengths of beaded chain. Attach a chain connector to the end of each. You'll need 9 lengths.
 * 2) Chain 5  1/2 inchwashers to one leg of a binder clip. This piece will be called a weight. Make 5 weights
 * 3) Chain two binder clips through one leg each. The chain length between them should be about 1 to 2 inches. This piece is called a connector. Make 4 connectors.

Building the mobile (ACTIVITY)

 * 1) Stage 1 Mobile: Take a rod and place a weight on each end. All weights must be attached as close to the rod ends as possible. This facilitates measuring distances along the rods. Balance it by placing a connector somewhere along the rod between the two weights.
 * 2) Stage 1 Analysis: See picture for the stage 1 mobile. Where do you attach the connector to the rod? This is the only choice you have. The weights must be on the ends of the rod, but you can move the connector in the middle to the left or the right until you get the rod to balance. How long is it from the connector to the weight? How far is it from the connector to the other weight? (When measuring distances, remember to measure to and from the center of the binder clip.) Draw the mobile including the lengths and weights.
 * 3) Stage 2 Mobile: Take a new rod. Hang stage one from the end of the new rod. On the other end of the new rod hang another weight. Attach a connector to the new rod. Adjust the connector position until both hanging rods are balanced.
 * 4) Stage 2 Analysis: Where did you attach the new connector? Did you have to change anything about the stage one mobile hanging from the new rod? Draw the mobile including the lengths and weights.
 * 5) Stage 3 Mobile: Take a new rod. Hang the stage two mobile from the end of the new rod. On the other end of the new rod hang another weight. Attach a connector to the new rod. Adjust its position until the new rod is balanced. Draw the mobile including the lengths and weights.

How it Fits to Standard

 * Forces are pushes and pulls. Motion is a change in position.
 * Carry out investigations by using instruments, observing, recording, and drawing evidence-based conclusions.

Follow Up
Look at the pictures drawn and consider the weights on the end of each rod and the distance to each weight. Draw each rod as a lever. Which class is each rod? Where is the fulcrum? Where is the resistance and the effort (which are interchangeable here)? What math relationship is there for the amount of weight and the distance from the fulcrum?