University of Virginia Physics Department Levers A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.10

Objectives

Students will

• construct a lever;
• understand how a lever functions;
• explain the concepts of lever arm, load, fulcrum, effort and mechanical advantage;
• investigate the applications of a lever.

Motivation for Learning

Discrepant Event-Lift the Teacher

Materials

• Cube of wood (4"x 4")
• Wood board 2" by 6" (8 feet long)

Procedure

1. Construct a lever, using the cube as the fulcrum and the wood board as a lever arm.
2. Initially place the fulcrum at the midpoint of the board. Stand on one end, and challenge a student to lift you off the ground by pushing the other end with a hand. If this cannot be accomplished by one student, ask for more volunteers until they are able to lift you. BE CAREFUL THAT YOU DON'T FALL OFF!!!
3. Ask the students what would happen if the position of the fulcrum were changed. Where should it be moved to require the least amount of effort?
4. Vary the position of the fulcrum and repeat the test. The closer the fulcrum is to the student, the more difficult it will be for them to lift you.

Background Information

The lever is an example of a simple machine, a device that changes the amount of force that is required to do work. A lever is a bar, or lever arm, which pivots about a fixed point, or fulcrum. The simplest type of lever uses a force exerted on one side of a fulcrum to lift a load on the opposite side.

The mechanical advantage of a simple machine is the magnitude by which it decreases the required effort. The mechanical advantage of a lever is calculated using the respective distances from the fulcrum to both the load (the load arm) and the point of effort (the effort arm). For example, if a lever's mechanical advantage were 2, half the force would be necessary to lift an object.

From this equation, it can be seen that as the distance from the fulcrum to the effort increases, so does the advantage of the lever. In other words, the farther the effort is from the fulcrum, the easier it will be to lift an object.

In this experiment, the students will use a spring scale to measure the required effort to lift specific loads. This effort will be measured in units of force, newtons. Before beginning the experiment, familiarize the class with the spring scale. The spring scale will hang from one side of the lever arm (see student procedure). It is important to note that initially, without any additional effort, the spring scale provides a downward force due to gravity. This force should be measured and recorded by the students before beginning the experiment, as outlined in the procedure. However, the scale reading during the experiment will not account for this force. Therefore, this amount should be added to each student's results to obtain the total effort exerted on the effort arm.

Student Activity

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Materials

2 Spring scales (10 N or 1 kg maximum)	Gravel or Sand (240 g) to fill plastic bottle
3 Rubber bands, medium size	Plastic spoons
½ meter stick or ruler	Water to fill excess space in plastic bottle
Binder clip, medium	Triple beam balance or digital scale
Dowel, at least 25 cm long, 0.5 cm diameter	Masking tape (one inch wide)
Pencil eraser, wedge-shaped, to fit on end of dowel	Black ball point pen
Plastic bottle with screw eye in lid (250 mL)--can be ordered from Delta Education Catalog (1-800-258-1302); item number 420211232

Procedure

1. Fill the plastic bottle with gravel. Mass it using the balance. If the mass is less than 240 g, add water to increase the mass.
2. Attach one rubber band to the screw eye of the bottle and one to the top hook of the spring scale. This will allow the bottle and scale to hang from the ruler.
3. Cover the inch measurements on the ruler with masking tape. Using a meter stick or a second ruler to measure, mark increments of 5 cm along the masking tape from end to end.
4. Secure the dowel rod to a desk or table top with masking tape and possibly a book or other heavy object.
5. Clip the binder clip to the midpoint of the ruler from the bottom side. Flip up the metal clips, so that they curve down. Slide this loop onto the dowel rod.
6. Attach the eraser to the end of the dowel rod, so that the binder clip cannot slide off.
7. To measure the force of the spring scale itself, attach one spring scale to the bottom of another. Read the measurement off of the top scale, and record it in the data sheet.
8. Hang the load from the spring scale and record the amount of force that is required to lift it without the lever.
9. Hang the load on the lever arm, 10 cm from the fulcrum (binder clip). Attach the spring scale to the opposite end of the lever arm, 2.5 cm from the fulcrum. Pull down on the spring scale to raise the load until the lever arm is parallel to the surface. Record the required force on the data sheet.
10. Repeat step 9 with the spring scale at five more locations: 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm from the fulcrum. Record the results on the data sheet.

Data Sheet

To print out the Data Sheet only, click here.

Spring Scale Force Alone (N) Position of Effort (mass) Effort Reading (N) Total Effort (Reading + Spring Scale Force) (N) 2.5 cm     5 cm     10 cm     15 cm     20 cm     25 cm     Questions 1. Plot your results on graph paper. Label the x-axis for distance from the fulcrum by units of centimeters. Label the y-axis as total effort in newtons. (Your graph should resemble the example below) Connect the data points to make a curve. Use this plot to answer the following questions.   2. How much force would be required to lift the load (at 10 cm) if the effort were 13 cm from the fulcrum?     3. If 7.5 N of force were required to lift the load, how far from the fulcrum was the effort?     4. If the load were placed 20 cm from the fulcrum instead of 10 cm, how would your data change? Would your curve look any different? Explain.     5. Give three examples of levers that you could find in your home or school. Identify the effort, load and fulcrum.

Students with Special Needs

Some students may have difficuilty maniupulating the objects necessary for the assembly of the apparatus. This activity can be done with partners or in small groups.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.