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Scientific Experiments

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Experimental Variables

It’s exciting to roll down a skateboarding ramp, especially if you’re going fast. The steeper the ramp, the faster you’ll go. What else besides the steepness of a ramp influences how fast an object goes down it? You could do experiments to find out.

What Is an Experiment?

An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. For example, the speed of an object down a ramp might be one variable, and the steepness of the ramp might be another.

Experimental Variables

There must be at least two variables in any experiment: a manipulated variable and a responding variable.

  • Variable  – A variable is anything that can affect the outcome of an experiment.
  • A manipulated variable is a variable that is changed by the researcher. A manipulated variable is also called an independent variable or the tested variable.
  • A responding variable is a variable that the researcher predicts will change if the manipulated variable changes. A responding variable is also called a dependent variable or measured variable. It is the one difference between the experimental group and the control group. .
  • Experimental Group  – This is the group of trials that we are testing, it includes our tested variable.

    Control Group  – The control group is used for comparison, it is identical to the experimental group except it doesn’t include the tested variable.

You can learn how to identify manipulated and responding variables in an experiment by watching this video about bouncing balls:

http://www.youtube.com/watch?v=Tsqg-43Gk8c&NR=1&feature=endscreen (9:21)

Q: If you were to do an experiment to find out what influences the speed of an object down a ramp, what would be the responding variable? How could you measure it?

A: The responding variable would be the speed of the object. You could measure it indirectly with a stopwatch. You could clock the time it takes the object to travel from the top to the bottom of the ramp. The less time it takes, the faster the average speed down the ramp.

Q: What variables might affect the speed of an object down a ramp?

A: Variables might include factors relating to the ramp or to the object. An example of a variable relating to the ramp is its steepness. An example of a variable relating to the object is the way it moves—it might roll or slide down the ramp. Either of these variables could be manipulated by the researcher, so you could choose one of them for your manipulated variable.

Experimental Controls

Assume you are sliding wooden blocks down a ramp (see Figure below ) in your experiment. You choose steepness of the ramp for your manipulated variable. You want to measure how changes in steepness affect the time it takes a block to reach the bottom of the ramp. You decide to test two blocks on two ramps, one steeper than the other, and see which block reaches the bottom first. You use a shiny piece of varnished wood for one ramp and a rough board for the other ramp. You raise the rough board higher so it has a steeper slope (see sketch below). You let go of both blocks at the same time and observe that the block on the ramp with the gentler slope reaches the bottom sooner. You’re surprised, because you expected the block on the steeper ramp to go faster and get to the bottom first.

Q: What explains your result?

A: The block on the steeper ramp would have reached the bottom sooner if all else was equal. The problem is that all else was not equal. The ramps varied not only in steepness but also in smoothness. The block on the smoother ramp went faster than the block on the rougher ramp, even though the rougher ramp was steeper.

This example illustrates another important aspect of experiments: experimental controls. A control is a variable that must be held constant so it won’t influence the outcome of an experiment. In the case of your ramp experiment, smoothness of the ramps should have been controlled by making each ramp out of the same material. For other examples of controls in an experiment, watch the video at the URL below. It is Part II of the above video on bouncing balls.

http://www.youtube.com/watch?v=LeqOQQRzCfo&NR=1&feature=endscreen (5:44)

Q: What other variables do you think might influence the outcome of your ramp experiment? How could these other variables be controlled?

A: Other variables might include variables relating to the block. For example, a smoother block would be expected to go down a ramp faster than a rougher block. You could control variables relating to the block by using two identical blocks.

Summary

  • An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values.
  • There must be at least two variables in any experiment: a manipulated variable and a responding variable.
  • A control is a variable that must be held constant so it won’t influence the outcome of an experiment.

Vocabulary

  • control : Variable in an experiment that is held constant so it will not influence the outcome.
  • experiment : Controlled scientific study of a limited number of variables.
  • manipulated variable : Factor that is changed, or manipulated, by a researcher in a scientific experiment; also called independent variable.
  • responding variable : Factor in an experiment that is expected to change, or respond, when the manipulated variable changes; also called dependent variable.

Practice

Do virtual experiments with objects sliding down ramps at the following URL, and then answer the questions below. For the y-axis, select Y pos (position of the object relative to the starting point), and for the x-axis, select time (time elapsed since the object started moving). From the controls, select one variable other than delta T to be your manipulated variable. Change this variable while controlling the others, and observe how it affects the movement of the object down the ramp.

http://www.physicslessons.com/exp3b.htm

  1. What are your manipulated variable and control variables?
  2. What is the responding variable?
  3. How does the responding variable change when you increase the value of the manipulated variable?

Review

  1. What is an experiment?
  2. Distinguish between the manipulated variable and the responding variable in an experiment.
  3. Why is it important for other variables in an experiment to be controlled?

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