Determining A Spring Constant

Fred Pannell                   Luther Burbank School
                               2035 N. Mobile Ave.
                               Chicago, IL 60639
                               312-804-3000

Objectives:

(For grade 8 or above)    
To determine the spring constant of a hanging spring.
To interpolate and extrapolate information from a graph.

Materials Needed:

The following materials are needed for each group of 2-4 students:
1) a spring support made from a 6" piece of 1" by 4" and a 18"by 3/8" dowel rod
2) a spring support is made by drilling a 3/8" hole in the 4" by 6" piece of 
    wood 4" from the length and 2" from the width. Insert the dowel rod and 
    secure with a little white glue or any wood glue
3) a clamp from which to hang the spring 
4) spring (10 to 15 cm in length)
5) meter stick or metric tape
6) set of masses (10-250g)
7) several objects of unknown mass (a bolt or nut between 50 and 150 g)
8) graph paper
9) Grease pencil (optional)

Strategy:

Part I
                                                          
1) Prepare table as shown:        |  f  |  stretch in cm |  
   f = force in newtons.          |     |                |             
                                  |     |                |
                                  |     |                |
                                  |     |                |
2) Hang the spring from the support. 
3) Select a mass from a set of mass weights (be sure that the mass will stretch 
   the spring at least cm).
4) Convert the mass to force in newtons.  Should you use a 250g mass (.25kg), 
   multiply its mass by 9.8 m/sec2 to get 2.4kgm/sec2 (2.4 newtons). 
5) Support the mass with your hand so that the  spring is not stretched and 
   record this position.  Slowly release the mass and, when the spring reaches 
   its lowest position, record this position. 
6) In your data table record under f, the force in newtons and in stretch in cm
   record the number of centimeters of stretch.               
7) Continue numbers 2 through 6 using four or five other masses.
8) Construct a graph from the data in your data table.
9) Compare your graph with other groups.  Are they similar or are they 
   different? 

Part II

1) Place an object of unknown mass on your spring and record the stretch.
2) Using the graph determine:
   a) The force in newtons of the object.
   b) The mass of the object.

Conclusions:

1) What can you determine from your graph and others?
2) What would the graph look like if you used springs of different materials, 
   lengths or/and strengths?

Evaluation:

A student's evaluation is based on the accuracy of their unknown masses.

References:

Science Laboratory Techniques  Rolland B. Bartholomew and Frank E. Crawley
Addison-Wesley Publishing Company.