Experimental Variables

There are many variables involved in a problem. These are physical quantities which

• are specific,
• can be measured directly with an instrument,
• or calculated from other direct measurements,

 

Examples of Directly Measured Variables
the length of a simple pendulum diameter of a ball time for 20 oscillations of a simple pendulum electric current passing through a metal wire.

Examples of Calculated Variables
average speed of a moving ball (v = s/t) frequency of a simple pendulum (f = 1/T) resistance of a metal wire (R = V/I)

Poor Examples (Should Not Be Used!)
size of a ball (volume, diameter, mass?) how fast a ball moves (time taken, instantaneous speed, average speed?) location person doing the experiment

Keep it Physics!
Although the following are measurable, and would form good variables for some scientific research questions, they are more related to biology, psychology, etc. Not Physics! number of times a person blinks per minute number of people visiting the Japanese Food Stall each day

Independent & Dependent Variables

In an experiment to be carried out, you need to identify two variables that are related to each other. These are called the independent variable and dependent variable. The independent variable affects the value of the dependent variable.

 

Examples of Independent Variable (IV)	Examples of Dependent Variable (DV)
Length of simple pendulum	Period of simple pendulum
Height of ball above the ground	Time taken for the ball to fall to the ground
Length of metal wire	Resistance of metal wire

Constants

While doing an experiment to investigate the relationship between two chosen variables, it is important to keep everything else unchanged.

We will normally try to identify at least two specific, measurable variables to be kept constant to ensure a fair experiment. These may be physical properties of objects in the experiment, or relate to performing the experiment itself.

Examples	Variable To Be Kept Constant
property of object	mass of pendulum bob used e.m.f. of battery used power of light bulb used voltage rating of light bulb used
experimental setup	angle of incline of ramp (from which a marble is rolled down)
initial conditions	height from which the ball is released temperature of water used

Note: these “constant variables” also need to be specific and measurable.

Creating an Experiment

When planning an experiment it is common practice that each variable must be clearly stated in words, and also assign a suitable symbol.

It is not necessary to give a symbol for constants.

Example: Defining your Variables for an Experiment
Independent variable (IV): • Length of pendulum, L Dependent variable (DV): • Period of pendulum, T Variables to be kept constant: • Mass of pendulum bob • Angle pendulum bob is displaced before being released

 

The variables can then be referred to by their symbol throughout the experimental plan.

e.g. we need only say
“plot a graph or T against L”

instead of
“plot a graph of period of pendulum against length of pendulum”

Basics

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