Materials Included In Kit

Additional Materials Required

Safety Precautions

These laboratory activities are considered safe. Follow all normal laboratory safety rules.

Disposal

All materials are reusable and require no disposal procedures.

Teacher Tips

• Enough materials are provided in this kit for one group of 2–4 students. The laboratory activity and its discussion will likely require two or more 50-minute class periods.
• Students enjoy studying the levers of the human body. The storage tube from this kit can be used to study the biceps muscle (force) in the forearm (Lever Class III). You will need a milk jug and a piece of rope (see Figure 5). The following demonstration dramatically illustrates the principles of the Class III lever system.
  
  

  How Strong—How Long?
  
  What happens to the force required to lift a load as the load is moved along the lever?

  1. Fill a gallon jug with water and screw on the cap.
  2. Tie a rope through the handle of the jug forming a loop that is large enough to slide along the arm (see Figure 5).
     {13848_Tips_Figure_5_Demonstrating the Class III lever in the forearm}
  3. Select a volunteer—someone who thinks he or she is strong! Have the volunteer place his or her elbow on the edge of the table or desk. Place the rope loop near to the elbow and have the volunteer support the jug with his or her arm extended over the edge of the table. Move the jug successively further out on his or her arm until the jug is at the wrist. Have the volunteer describe the difficulty in supporting the jug as “harder” or “easier” along the length of the arm. (The volunteer should note that it requires more force to support the jug as it gets further away from the elbow [fulcrum].)
  4. Now “extend” the volunteer’s arm by adding the mailing tube over the volunteer’s arm as illustrated. How far away from the elbow can the jug be placed and still be supported by the volunteer? Have the volunteer describe the muscle that is supporting the jug and what it is like as the jug gets further from the muscle.

Sample Data

Lever Class I Worksheet

Effect of moving the fulcrum along the lever while the load and force positions remain constant.

Answers to Questions

Lever Class I Worksheet

1. In a Class I Lever, where is the fulcrum when the force and load are equal?

   The fulcrum is equidistant between the force and load.

2. In a Class I Lever, what happens to the force required to lift a load as the fulcrum gets closer to the load? What happens to the mechanical advantage?

   The force required to lift the load decreases and the mechanical advantage increases.

3. When the fulcrum is very close to the load and the force is far from the fulcrum, how does the distance the force moves compare to the distance the load moves?

   The load moves a very short distance compared to the lever at the point of the force.

4. True or False? Defend your answers.
   1. Lever Class I system would be good for moving a heavy object a small distance with less force required than the load.

      True. The mechanical advantage is large when the fulcrum is close to the load.

   2. Lever Class I system would be good for moving an object with great speed.

      True. If the load is small and the fulcrum is close to the applied force, the load can be moved a great distance very quickly.

   3. A shovel is an example of a Lever Class I.

      If the hand positioned in the middle of the shovel is used as a fulcrum, it can be a Lever Class I.

1. Where would you place a load with this lever system to exert the least force to lift the load?

   When the load is closest to the fulcrum, the least force is required to lift the load.

2. Would Lever Class II be a good system for lifting a heavy load with minimal force? Explain. How might the position of the lever be a problem with a Class II lever?

   Yes. The lever is very capable of having a high mechanical advantage. Getting the actual lever under the load can be a problem.

3. Would Lever Class II be a good system for moving a load a long distance? Explain.

   No. When the load is close to the fulcrum, the force moves a great distance while the load hardly moves any distance. The lever can have a good mechanical advantage, but has no speed or distance advantage.

4. Think of at least one common item that illustrates a Lever Class II system and explain how it works. What are the advantages and disadvantages of using the item in doing work?

   A wheelbarrow is a Class II lever system. Its good mechanical advantage allows lifting a heavy load with little relative force. The load, however, is not lifted very high off the ground if this were desired.

Lever Class III Worksheet

1. What happens to the force required to lift the load as the force gets further from the load?

   The force required to lift the load increases.

2. What happens to the mechanical advantage as the force gets closer to the fulcrum?

   The mechanical advantage decreases as the force gets closer to the fulcrum.

3. When the force is close to the fulcrum and a load is lifted, how does the distance the force moves compare to the distance the load moves? When might such an arrangement be advantageous?

   The distance the load moves is much greater than the distance the force moves. Such an arrangement can use this distance traveled to gain speed.

4. For each diagram, determine what lever class is illustrated an how the lever system is advantageous.

Introduction

A wheelbarrow, a shovel, a hammer, and nearly all tools are examples of lever systems in action. We utilize levers every day and have numerous examples in our own bodies. How do these simple machines work?

Concepts

• Force
• Load
• Fulcrum
• Lever class

Background

Levers are rigid objects, usually in the shape of a bar, that can turn on one point or axis called the fulcrum. A lever is used for the transfer and modification of force and motion. The movement of objects can be made faster or slower, longer or shorter, easier or harder, and can occur in various patterns. In a lever system, the lever itself is always rigid—like a bar, rod, plank, or other inflexible object. The load is whatever is being moved—a rock, a load in a wheelbarrow, or other heavy object. The force is anything capable of doing mechanical work; it may be a person, a spring, a motor, a jet or any other item that can exert a force on the lever itself.

Lever systems in action are useful in gaining speed, distance, precision, or mechanical advantage. Mechanical advantage is defined as the ratio of force output to the force applied. Each lever system has its own unique properties and has trade-offs between mechanical advantage and other properties. In general, there is a reverse relationship between mechanical advantage and both the amount and speed of movement, but there is no necessary relationship to precision.

The three basic lever classes are diagramed below. They are arbitrarily called Classes I, II and III. Some texts call them A B, and C.

Materials

Safety Precautions

These laboratory activities are considered safe. Follow all normal laboratory safety guidelines.

Procedure

Lever Class I: Fulcrum Between the Force and Load

1. Use the lever materials to set up a Class I lever system like that shown in Figure 2.
   {13848_Procedure_Figure_2_Lever Class I}
2. Use slotted weights to place a load on one end of the lever. Be sure to hold the lever in place while adding the slotted weights.
3. Use a spring scale to measure the force needed to hold the lever system in balance (level). The spring scale will need to be held off the edge of the lab table.
4. Without changing the size of the load or the position of the load and force, move the fulcrum to different positions along the lever. Measure the force required to hold the load at each position.
5. Record the distances D_f and D_l along with the force measurements for each fulcrum position on the Lever Class I Worksheet.
6. Calculate the mechanical advantage for each position tested and record the calculations on the worksheet.
   {13848_Procedure_Equation_1}
7. Answer the questions on the Lever Class I Worksheet.

1. Use the lever materials to set up a Class II lever system like that shown in Figure 3.
   {13848_Procedure_Figure_3_Lever Class II}
2. Use slotted weights to place a load on the lever between the fulcrum and force. Be sure to hold the lever in place while adding the slotted weights.
3. Use a spring scale to measure the force needed to hold the lever system in balance.
4. Without changing the size of the load or moving the position of the force or fulcrum, move the load to different positions along the lever. Measure the force required to hold the load level at each position.
5. Record the distances Df and Dl along with the force measurements for each load position on the Lever Class II Worksheet.
6. Calculate the mechanical advantage for each position tested and record the calculations on the worksheet.
7. Answer the questions on the Lever Class II Worksheet.

1. Use the lever materials to set up a Class III lever system like that shown in Figure 4.
   {13848_Procedure_Figure_4_Lever Class III}
2. Use slotted weights to place a load on the end of the lever far away from the fulcrum with the force between the fulcrum and load. Be sure to hold the lever in place while adding the slotted weights.
3. Use a spring scale to measure the force needed to hold the lever system in balance. (The top of the fulcrum will need to be held in place. See Figure 4.)
4. Without changing the position of the fulcrum or load, move the position of the force along the lever. Measure the force required to hold the load level at each position.
5. Record the distances and force measurements on the Lever Class III Worksheet.
6. Calculate the mechanical advantage for each position tested and record the calculations on the worksheet.
7. Answer the questions on the Lever Class III Worksheet.

Student Worksheet PDF

13848_Student1.pdf