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In a force-extension graph why is the extension plotted on the x-axis and the force on the y-axis?

• Logically, the extension of an object (spring,wire,etc.) is dependent on the force applied to it. Therefore, in a force against extension graph, the force should be plotted on the x-axis (being the independent variable) and the extension on the y-axis. But why do all the physics books have it the other way round?

• Answer:

  In an actual experiment, either F or x could be the independent variable depending on how you set up the measurement. If you load the spring by hanging weights off the end and measure the net stretch, then F is independent and x is dependent. If on the other hand you load the spring by squishing it by a certain number of turns of the handle in a vice, with a force transducer to measure how much the spring resists, then x is independent and F is dependant. Of the two, x is more natural for most theoretical physics because x also tends to be the independent variable for some larger problem. For example, if you're looking at the oscillations of a mass-spring system, it's most natural to write [math]F=-kx=ma=m\ddot{x}[/math]