LAB I: ELASTICITY

Problem: What is the relationship between the force applied to a spring and the stretch of the spring?

NOTE: DO NOT BEGIN YOUR EXPERIMENT UNTIL EACH PERSON IN THE GROUP HAS READ THE BACKGROUND AND ANSWERED THE BACKGROUND QUESTIONS.

Background and Inquiry: Today you will study some properties of elastic bodies. An elastic body has the ability to regain its original form after it is stretched. A spring is one example of an elastic body. Observe the properties of the spring by pulling slowly on the spring and observe the tension in the spring. DO NOT PULL THE SPRING TO FULL EXTENSION!! If an elastic body is stretched beyond a point called the elastic limit, it will no longer retain its original properties. By feeling the tension on the spring discuss with your group how changes in force may the affect the amount of stretch observed.

Many things in nature have elastic properties. A rubber band is one example. A spring is another. Can you think of others? Discuss with your group several other examples of elastic bodies.

Take the rubber band and stretch it being careful not to extend it too far. Have each person in the group observe the properties of the two elastic bodies (spring and rubber band). Discuss any differences in their properties.

Today you will study the elastic properties of a spring and two different types of rubber band. Your problem today is to determine the relationship between the force needed to stretch an elastic body and the length it stretches. For your hypothesis try to predict the type of behavior you expect the spring and rubber band to follow as you change the amount of force (mass weights) applied to the spring. For example, would you expect if you double the weight the spring will stretch twice as much? Or three times? What type of relationship do you predict will occur?

Background Questions:

1) What is an elastic body?
2) Give 4 examples of elastic bodies.
3) Do you think the elastic properties of all elastic bodies are the same? Why?

Hypothesis: State your hypothesis to the problem. Justify your reason. .

Materials: ruler, spring, set of weights, ring stand, two different types of rubber band

Procedure:
1) Copy Table I, Table II, and Table III into your lab notebook.
2) Set up the equipment as shown in in class.

3) Starting with the smallest mass, measure the amount the spring is stretched (difference in length between spring without mass and with mass). See your class notes on how to measure this difference. Use additional masses as shown in Table I below. For each mass measure the the stretch. DO NOT EXCEED 500 GRAMS FOR THE SPRING.

3) Repeat step 3 using the rubber bands, instead of the spring.

Results:

Note: To simplify this lab we will only use mass in your tables and graph. See your class notes for clarification.

TABLE I

SPRING

TABLE II

RUBBER BAND 1 (THIN)

TABLE III

RUBBER BAND 2 (THICK)

Graph the data from Table I and Table II drawing two graphs, both on the same set of axis. Plot stretch (cm.) on the y-axis and mass (grams) on the x-axis. Make sure to label each axis, and each plot. Measure and record the slope of each plot Give the completed set of graphs a title.

Discussion:

Be sure to include the following:
1) What type of mathematical relationship is demonstrated in Graph I? Why?
2) Does graph II and III show more than one relationship? For example, does the graph start off linear then change? If so explain why.
3) What are the variables in this experiment? Which is the dependent variable, which is the independent variable?
4) How are the variables changing with relationship to each other?
5) What are some factors that are held constant in this experiment?
6) Can you predict what the stretch would be in Table I if 350 grams were attached to the spring? Why?
7) Can you predict what the stretch would be in Table I if 800 grams were attached? Why? or Why not?

Applications:

1) How can your graph be used to find the force of an unknown object?
2) Design an experiment to predict if a spring has been over-stretched and is deformed.
3) Research Hooke's Law. How does it relate to this lab?