Designing a Lab Investigation An overview

First is the Question The investigative question looks at how the manipulated variable affects the responding variable. The manipulated variable is what the experimenter is intentionally changing. The experimenter decides how much to change the conditions of the manipulated variable. There should only be ONE manipulated variable per investigation.

Question cont’d The responding variable is what the experimenter is measuring. The question should NOT be a yes/no question, but instead should ask about the relationship between the two. “How does the manipulated variable affect the responding variable?” is the general format to use.

Examples: Manipulated & Responding How does the pH of rainwater affect the percentage of seed germination? How does the mass of the pendulum bob affect the pendulum’s period? How does the temperature affect the rate that sugar dissolves in water? How does the force applied to a toy car affect the distance the car travels?

Next comes the hypothesis. Here the experimenter predicts how the manipulated variable affects the responding variable. The hypothesis should follow the “If … then…” format. This should predict how specific changes in the manipulated variable will cause the responding variable to change. Do not just say “ The responding variable will be affected.” Say how.

Examples: If the pH of rainwater is more acidic, then a lower percentage of seeds will germinate. If the mass of the pendulum bob increases, then the pendulum’s period will be unchanged. If the temperature of the water increases, then sugar will dissolve at a faster rate. If a greater force is applied to a toy car, then the car will travel a larger distance.

The sentences after the hypothesis should offer reasons for the prediction. In most of the lab write-ups we do, the reasons should come from information we have covered in class or read in the textbook. You should clearly connect the reason to the same variables in the question. You should cite the source of your information and clearly tell what information your source gave you.

After the Question/Hypothesis comes the Procedure.

Procedure This should be a list of NUMBERED steps that clearly instruct how to carry out the lab. It should start with a list of important materials. The quantities of materials should be given in the step of the procedure, not just assumed because a quantity is listed in the materials.

Procedure There should be only one variable that is manipulated. The procedure should clearly outline how this variable is changed – preferably with numerical values. The responding variable should also be clearly stated – what EXACTLY is being measured. So.. the responding variable must be MEASURABLE. Do not just say “ Record data.” Ever! Really, do NOT say that.

Procedure The steps should identify what variables are going to be controlled (kept the same). The procedure should also include an experimental control that is the condition as close to possible to normal (often the manipulated variable value is zero).

Procedure Sometimes writing the procedure is a balancing act. You want your steps to be clear and precise, but you do not want them to be overly long and complicated. So skip trivial steps like “Gather materials” unless they are located in an unusual place or must be treated in a special way.

1. Attach a loop of string to a skateboard. Place the bricks on the skateboard. 2. Using masking tape, mark off a one-meter distance on a level floor. Label one end “Start” and the other “Finish.” 3. Attach a spring scale to the loop of string. Pull it so that you maintain a force of 2.0 N. Be sure to pull with the scale straight out in front. Practice applying a steady force to the skateboard as it moves. 4. Find the smallest force needed to pull the skateboard at a slow, constant speed. Do not accelerate the skateboard. Sample Procedure Experimental Control Presented as a numbered list

6. Add 0.5 N to the force in Step 5. This will be enough to accelerate the skateboard. Record this force on the first line of the table. 7. Have one of your partners hold the front edge of the skateboard at the starting line. Then pull on the spring scale with the force you found in Step When your partner says “Go” and releases the skateboard, maintain a constant force until the skateboard reaches the finish line. Specifically says to “record” Includes controlled variables

9. A third partner should time how long it takes the skateboard to go from start to finish. Record the time in the column labeled Trial Repeat Steps 7, 8 and 9 twice more. Record your results in the columns labeled Trial 2 and Trial Repeat Steps 7, 8, 9, and 10, using a force that is 1.0 N greater than the force you found in Step Repeat Steps 7, 8, 9, and 10 twice more. Use forces that are 1.5 N and 2.0 N greater than the force you found in Step 5. Record your results. The responding variable is clearly explained Tell to repeat for at least three trials of the same condition. Manipulated variable clearly changed

After Procedure comes your Data. Here is where you present your results so others can see what you found out. Starts with a Data Table that presents the raw data you recorded and relevant calculations. The table should be easy to understand – organized and labeled. Include UNITS!

Table 1: Times, speeds, and acceleration of a skateboard traveling 1 m based on varying forces. Table is clearly explained and identified. Manipulated variable given – with UNITS. Responding variable – all trials and averages – with units Relevant Calculations

Table 1: Leaf floating time – Data from the entire class. Trial Light intensity (volt, cm) Individual floating times (minutes)Average floating time (minutes)