Why do we do masses on springs?

This question arises from the 3rd year dynamics paper I’m teaching at the moment. How come in lectures we only ever cover simple examples of things (in the context of this paper, moving things), like a mass bouncing on a spring, rather than realistic examples, like a washing machine or aircraft engine.

It’s a fair question – it’s not just this paper, or even physics, where we present examples so simplified they seem to have no everyday use. I mean, when do you ever, in practice, build an electrical circuit with a battery and two light bulbs in parallel, or drop a ball from a first floor window and need to know how quickly it hits the ground?

 

The reason is, that the simple examples are used to help students grasp the underlying concepts. It is the concepts that is what physics is about. I cannot think of a possible application for hanging a mass on a spring and letting it bounce (our simple system beloved of physics lecturers)  but I can think of numerous examples of where resonance is important. Such as vibration in jet engine turbine blades (you had better hope that those designers know about resonance), tuning a radio (in this case resonance isn’t bad – it’s useful), musical instruments, car suspension systems, response of buildings to earthquakes, etc etc etc. And the mass on the spring makes a perfect illustration of the concept of resonance.

If you don’t get what is happening when when a mass bounces up and down on the end of a spring, please don’t try to design an aircraft.

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