University of Virginia
Physics Department

## Experimental Design

A Physical Science Activity

2003 Virginia SOLs

• PS.1

Objectives

Students will:

• Use a metric ruler to determine distance;
• Record data from an experiment;
• Identify independent variables, dependent variables, constants, controls, and repeated trials;
• Draw valid conclusions after analyzing data.

Motivation for Learning

Demonstration-Discrepant Event

Materials

• Sugar
• Ice water
• Room temperature water
• Hot water
• Beakers
• Spoon
• Stopwatch or clock with second hand

Procedure

1. Prepare three beakers with equal amounts of ice water, room temperature water, and hot water.
2. Ask students to hypothesize about which beaker of water will dissolve a teaspoon of sugar the fastest.
3. Ask for three student volunteers.
4. Give student #1 a level teaspoon of sugar and ask him to stir it into the ice water until it dissolves.  Use the clock or stopwatch to determine the number of seconds required to completely dissolve the sugar and record that number on the chalkboard.
5. Repeat with student #2 and room temperature water.
6. Repeat with student #3 and hot water.
- Note:  It is important that each student stir the water at the same rate as the other two.

Background Information

An experiment is an organized series of steps used to test a hypothesis.  Experimental design is a specific set of directions for designing and carrying out an experiment, so that the results are as valid as possible.  Experimental design seeks to eliminate experimental error and to insure that the results are due to the factor being tested.

The following vocabulary is used in experimental design:

• INDEPENDENT VARIABLE (IV)– The factor controlled by the experimenter.  This might also be described as the change made by the experimenter on purpose.  It is sometimes called the manipulated variable.
• LEVELS OF THE INDEPENDENT VARIABLE (DV)– A list of the changes that are made to the independent variable.
• DEPENDENT VARIABLE – The factor which changes because of what the experimenter does.  The dependent variable is the change that occurs because of what the experimenter does.  It is sometimes called the responding variable.
• CONSTANTS – Factors in the experiment which do not change throughout the entire experiment.  Controlling constants is very important so that the experimenter may be assured that the changes are due only to the changes in the independent variable.
• CONTROL – The part of the experiment to which no changes are made, so that the results can be compared.
• REPEATED TRIALS – The number of times that the experiment is done.

For the experiment described above:

• INDEPENDENT VARIABLE – water temperature
• LEVELS OF THE INDEPENDENT VARIABLE – cold, room temperature, hot
• DEPENDENT VARIABLE – the time needed for the teaspoon of water to dissolve
• CONSTANTS – water source, amount of water, speed of stirring, amount of sugar, type of sugar
• CONTROL – no control was described in this experiment; however, if more water temperatures were used, the room temperature water could be considered the control
• REPEATED TRIALS – only one trial was described, but stress to students that the greater the number of trials, the more valid the results.

In this activity, the students will use experimental design to determine the relationship between the distance between supports and the number of pennies needed to break a strand of uncooked spaghetti.  Textbooks are used for supports because they are readily available.  One strand of spaghetti is suspended between the two stacks of textbooks and a “bucket” is hung on the spaghetti.  Plastic three-ounce bathroom cups make ideal buckets when a hole is punched on each side and a handle made from a twist-tie.  The students will then place pennies, one at a time, into the cup until the spaghetti breaks.  As the distance between the stacks of books is increased, the number of pennies needed to break the spaghetti will decrease.  At least three trials should be carried out for each distance.  The data will be recorded in a data table and the average number of pennies will be found for each distance.

For the experiment in the student activity:

• INDEPENDENT VARIABLE – The distance between the stacks of books.
• LEVELS OF THE INDEPENDENT VARIABLE – 10 cm, 15 cm, 20 cm
• DEPENDENT VARIABLE – The number of pennies needed to break each strand of spaghetti
• CONTROL – There is no control in this experiment
• CONSTANTS –
• Diameter of spaghetti
• Brand of spaghetti
• Age of spaghetti
• Type of coin used (all pennies)
• Cup used as bucket
• REPEATED TRIALS - 3

### Student Activity

Materials (for each lab group)

• Ruler
• Eight textbooks
• Paper cup with handle
• Approximately 75 pennies
• 9 strands of spaghetti

Procedure

1. Use the ruler to place the two stacks of books 10 cm apart.
2. Gently hang the cup from one strand of spaghetti.
3. Place the spaghetti on top of the books to form a bridge.  The cup should be hanging down between the stacks of books like a bucket.
4. One at a time, add pennies to the cup until the spaghetti breaks.
5. Record the number of pennies needed to break the spaghetti in Data Table 1.
6. Repeat this procedure 2 times and record the number of  pennies in the data table.
7. Now use the ruler to place the stacks of books 15 cm apart.
8. Repeat the above procedure three times and record the number of pennies in the data table.
9. Finally, use the ruler to place the stacks of books 20 cm apart.
10. Repeat the above procedure three times and record the number of pennies in the data table.
11. Cleanup - be sure that all pieces of spaghetti are placed in the trash can - not in the sinks or on the floor! ! !

Data Sheet

 Distance Between Books Number of Pennies Required to Break Spaghetti Trial #1 Trial #2 Trial #3 Average 10 cm 15 cm 20 cm

Experimental Design

Fill in the following:

• Independent Variable

• Levels of the Independent Variable

• Dependent Variable

• Control

• Constants

• Repeated Trials

Extensions

Have motivated students try stating a different independent variable for a similar experiment.  For instance, they could keep the distance between books constant and try varying the thickness of the spaghetti strands.  Be sure to stress the importance of changing only one factor in each experiment.

Students with Special Needs

For students with processing difficulties it may be helpful to read one step at a time and have all groups complete that step before going further.  Then the teacher could be sure that the students are doing what they should and getting proper results.  Another strategy would be to have students physically check off the procedure steps as they complete them.

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

Assessment

1. Make a statement about how the distance between the stacks of books affects the number of pennies needed to break the spaghetti.

2. Why did you do three trials and find the average?

3. Why is it important to use the same type of spaghetti for each trial?

4. Why are constants important in an experiment?