Designing Experiments SelfGuide

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Where do scientific experiments come from?

In some labs, you will be given a scientific problem to solve by designing your own experiment. If that is the case, go on to PreLab Question #1 (below). But if you have been asked to come up with your own scientific problem, you need to do some thinking before beginning to answer the PreLab questions:

A scientific problem is something you don’t understand but you can do an experiment to help you understand. Scientific problems are usually based on observation of scientific phenomena. Here is some advice to help you identify a scientific problem you can address by designing your own experiment.

1) Find a topic:

A topic is a relatively specific area of knowledge, or subject, you will be working in, such as smoking and lung cancer, sexual selection in birds, gravity, Newton's Laws of Motion, properties of water, etc... If you have been given a topic, you may need to narrow it, to identify a more specific topic within the broader one. This can make it easier to work with. If you are supposed to choose your own topic, do some brainstorming about things you have learned about in your science course that was particularly interesting for you, something you’d like to know more about. Write down some possible topics and choose the one that seems most interesting to you.

2) Identify a problem within the topic:

The problem is something you’d like to know more about, a question you’d like to answer. Questions can come from many different sources: from lectures or textbooks, from an experiment you have done that raised other questions, from articles you’ve read in scientific journals or even newspapers and magazines. To identify a scientific problem, then, you can find sources that relate to your topic and look to see what problems are raised in your search. Write down the problems that you find. Choose one that would be interesting to solve and that is feasible for you to solve. Now you are ready to answer the following PreLab questions:

PreLab: questions to answer before doing the lab

1. What is the problem? Describe the problem in your own words. Be sure that your description includes known factors (information about the problem given to you in the lab in a problem statement, for example) and unknowns (what you need to find out in order to solve the problem). Then restate the problem in the form of a question or questions that will guide your research.


Background for the experiment:
Lori is on her school's volleyball team. Because she and her teammates spend a lot of time doing jumping exercises, she has become interested in the standing vertical leap, which plays an important role not only in volleyball but also in basketball. Lori knows that success in both sports depends to a large extent on an athlete's ability to jump higher than opposing players. She wants to design an experiment in biomechanics that will help her determine what jumping strategies athletes can use to jump their highest.

What is the problem?
The standing vertical jump plays a critical role in several sports, especially volleyball and basketball. In volleyball, players must spike the ball and block spikes by the opposing team by leaping from a standing position. In basketball, players often shoot under the basket and rebound from a standing position. In both sports, the effectiveness of the players can be increased by being able to jump higher. The problem, though, is that many players don’t know how to make the best use of biomechanics to jump their highest. This is an experiment that I hope will provide information that can help athletes, including myself, jump higher.

What I know is that there are, generally speaking, two approaches to vertical jumps from a standing position. The first is called the squat jump. You begin the jump in a modified crouch or squat with the knees bent and then spring from that position. In the other approach to jumping, you begin the jump with a downward movement of the body and arms that leads to an upward spring. The unknown is which of these two approaches allows the jumper to jump higher.

My question is: Which of these two approaches to the vertical standing jump, the squat jump and the countermovement jump, provides the biomechanics that result in a higher jump?

2. What do you know about the science of the problem that could help you answer your research question? State the scientific concept (see below for definition) that the lab is about (something like the quantization of energy, photosynthesis, or momentum conservation). Write down information you can find about the concept that might be useful in answering your research question (check lab manual, textbook, class notes, handouts, etc.). Note any citations of sources you use. Go to Citations and References under Resources of the web version of this document for more help.

Scientific Concept:
Most science labs are designed to help you learn about a scientific concept. If you are having trouble identifying the scientific concept this lab is about, check the title of the lab in the lab manual and read the introduction to the lab in the manual. It will be something like photosynthesis, quantization of energy, or momentum conservation.

Write down the scientific concept. Then write down what you know about the concept based on the lab manual, textbook, class notes, and handouts. Don’t try to make it pretty; just keep writing. Get as much down as you can. Because the point of the lab is to learn about the scientific concept, it’s important to state what you already know.


What do you know about the science of the problem that could help you answer your research question?
I found a web site and an article that helps me to understand the standing vertical jump. The scientific concept involved in understanding the standing vertical jump is Newton’s third law of motion: for every action, there is an equal and opposite reaction. Basically, when a person jumps, she is overcoming her body weight, the force of gravity that holds her on the ground. The jumper applies force against the surface she is jumping from and that force results in the equal and opposite reaction of resisting gravity by elevating off the jumping surface.

Thus, the height of a jump may be understood in terms how much force is exerted against the jumping surface. Height is a function of acceleration, velocity, and time: how much time the jumper is in the air depends on the acceleration and speed of the jump. The greater the force against the jumping surface, the greater the acceleration and velocity of the jump and the longer the jumper is in the air. The length of time in the air correlates to the height of the jump—“hang time.” Thus, acceleration/velocity/time are key to the height of the jump. What this scientific concept suggests is that the biomechanics of a jumper that allow the jumper to apply the greatest force against the jumping surface will lead to the highest jump.

3. What is your hypothesis for the answer to your research question? Using what you know about the problem and the scientific concept of the lab, state a hypothesis (see below for definition), your best estimation of the answer to your research question. Then describe the reasoning that led you to your hypothesis, using what you know about the scientific concept as a basis for your reasoning.

A hypothesis is a scientist's best estimation, based on scientific knowledge and assumptions, of the results of an experiment. It usually describes the anticipated relationship among variables in an experiment. Since dependent variables "depend" on independent variables, there has to be a relationship between the two. The anticipated relationship between the dependent and independent variables is the result you expect when one variable reacts with another.

A hypothesis typically leads to the crucial questions that must be addressed in the lab report: did you find what you expected to find? why or why not? The point of an experiment is to test the hypothesis. Write or sketch your hypothesis, describing the relationship among the variables you listed .


What is your hypothesis for the answer to your research question?
I hypothesize that the countermovement jumps will be higher than jumps from the squat position. I further hypothesize that the faster the downward countermovement of the jump the higher the jump. The reasoning behind these hypotheses comes out of Newton’s third law of motion. If for every action there is an equal and opposite reaction, then it is reasonable to expect that the more force that is exerted against the jumping surface the higher the jump. And the downward movement preceding the actual jump would apply more force against the jumping surface than simply jumping, like a wound up spring, from a squat position.

It is also logical that more rapid countermovements will put more force on the jumping surface and thus will lead to higher jumps. Generally speaking, then, the greater the force a jumper can apply against the jumping surface, the greater the acceleration and velocity of the resulting jump, the more time the jumper is in the air, and, therefore, the higher the jump.

4. What variables can you use to test your hypothesis? A well-designed experiment needs to have variables. Look over your hypothesis, and identify the variables that you will be testing during your experiment: what you can measure or observe (dependent variables) and what you can manipulate in an experiment for the measurements or observations (independent variables). List your variables. Then describe, in words or in a sketch, the relationship among the variables as predicted by the hypothesis. (See below for definitions of underlined terms.)

A variable is what is measured or manipulated in an experiment. Variables provide the means by which scientists structure their observations. Identifying the variables in an experiment provides a solid understanding of the experiment and what the key findings in the experiment are going to be.

To identify the variables, read the lab procedure described in the lab manual. Determine what you will be measuring and what you will be manipulating for each measurement. The value(s) you are manipulating is called the independent variable (see definition below) and the value(s) you are observing/recording is called the dependent variable (see definition below). Write down the dependent and independent variables.In more advanced labs, you may have multiple variables (see definition below), more than one independent and dependent variable.

Multiple Variables:
It is possible to have experiments in which you have multiple variables. There may be more than one dependent variable and/or independent variable. This is especially true if you are conducting an experiment with multiple stages or sets of procedures. In these experiments, there may be more than one set of measurements with different variables.

Example: You are interested in finding out which color, type, and smell of flowers are preferred by butterflies for pollination. You randomly choose an area you know to be inhabited by butterflies and note all the species of flowers in that area. You want to measure pollination of flowers by butterflies, so your dependent variable is pollination by butterflies. The independent variables are flower color, type, and smell. You will need to specify relationships for each of these independent variables with the dependent variable.

Dependent Variable:
A dependent variable is what you measure in the experiment and what is affected during the experiment. The dependent variable responds to the independent variable. It is called dependent because it "depends" on the independent variable. In a scientific experiment, you cannot have a dependent variable without an independent variable.

Example: You are interested in how stress affects heart rate in humans. Your independent variable would be the stress and the dependent variable would be the heart rate. You can directly manipulate stress levels in your human subjects and measure how those stress levels change heart rate.

Independent Variable:

An independent variable is the variable you have control over, what you can choose and manipulate. It is usually what you think will affect the dependent variable. In some cases, you may not be able to manipulate the independent variable. It may be something that is already there and is fixed, something you would like to evaluate with respect to how it affects something else, the dependent variable like color, kind, time.

Example: You are interested in how stress affects heart rate in humans. Your independent variable would be the stress and the dependent variable would be the heart rate. You can directly manipulate stress levels in your human subjects and measure how those stress levels change heart rate.


What variables can you use to test your hypothesis?
Because I have two hypotheses, I need two sets of independent and dependent variables. For the first hypothesis (that the countermovement jumps will be higher than squat jumps), my independent variable will be the two different approaches to the biomechanics of jumping, the countermovement jump and the squat jump. These are independent variables because I will be able to manipulate them in my experiment. My dependent variables, what I will be measuring in the experiment, will be the heights of jumps by the subjects in my experiment.

For my second hypothesis (that the faster the countermovement the higher the jump), my independent variable will be the different downward speeds my subjects will use as they jump. The dependent variables will be the different heights of these jumps by the subjects in this experiment.

5. What experiment(s) could you use to test your hypothesis? Referring to the list of variables, brainstorm some experiments you could do that would allow you to manipulate variables so that you can make the measurements or observations necessary for testing the hypothesis.Your experiment may require control and treatment groups (see below for definition). Choose the experiment most likely to yield the results you need to test your hypothesis. List the materials and outline the methods you will use for your experiment. (Remember that you have to work with the materials and lab instruments available to you.)

Control and Treatment Groups:
A control group is used as a baseline measure. The control group is identical to all other items or subjects that you are examining with the exception that it does not receive the treatment or the experimental manipulation that the treatment group receives. For example, when examining test tubes for catalytic reactions of enzymes when added to a specific substrate, the control test tube would be identical to all other test tubes with the exception of lacking the enzyme. The treatment group is the item or subject that is manipulated. In our example, all other test tubes containing enzyme would be part of the treatment group.


What experiment(s) could you use to test your hypothesis?
Experiment #1. I will use 6 players from my volleyball team. I will begin by training the players in the two approaches to jumping biomechanics, giving them opportunities to practice each approach. Then each player will make a series of five paired jumps related to the independent variable. Three players will do pairs consisting first of a squat jump and then a countermovement jump. The three other players will have the opposing pairing, first countermovement jump and then squat jump. (This is to be sure that fatigue doesn’t affect the results.) The players will make their jumps in a standard set up used for jumping exercises: a suspended board with measurement lines marked on it that players will touch at the height of their jump. I will record the height of each of the jumps.

Materials needed: vertical jump measurement set-up

Experiment #2. I will use the same 6 players. I will ask each one to make five countermovement jumps, each jump in the series consisting of a downward thrust of a different speed, which I will measure according to the time it takes. I will train the players to jump to a number count from the moment they begin the downward movement to the point their feet leave the jumping surface. Half the players will start with a slow downward movement, the first to a count of 5, second 4, and so on to a quick count of 1. The other half will proceed from fast to slow. In order to make more accurate measurements of the speed of the countermovements, I will videotape the jumps and time each of them (from the beginning of the downward movement to the point the feet are off the surface) with a stopwatch while viewing the video later. I will record the height of each of the 5 jumps in the series.

Materials needed: vertical jump measurement set-up; camcorder; stopwatch; VCR.

Linthorne, Nick. "Standing Vertical Jump." (3 January 2004).

Linthorne, Nicholas P. "Analysis of standing vertical jumps using a force platform."
American Journal of Physics 69 (2001): 1198-1204.


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