Chapter 5: Stretching and compressing
A spring is the simplest machine there is. Pull it and it pulls back. In this chapter you find the rule that connects how hard you pull to how far it moves — and you find where that rule stops working.
The chapter problem
The spring that stopped behaving. You hang weights on a spring and measure how far it stretches. Up to a point, each extra newton stretches it by the same amount. Then it starts stretching further for the same extra force — and past that, it does not return to its original length at all.
By the end of the chapter you can find the spring constant from your own measurements, say from the graph where the rule stopped holding, and explain what the spring is doing after that point.
By the end of this chapter you can
- state Hooke's law and use to find a force, a spring constant or an extension;
- describe what the limit of proportionality is, and what a force-extension graph looks like on either side of it;
- find the spring constant of springs joined in series and in parallel, and say why one combination is stiffer than the other;
- calculate the energy stored in a stretched spring.
Where it is used
Springs and suspension, measuring instruments, trampolines and car crumple zones — and any design where you want a material to bend in a way you can predict.
Your turn to connect it
Where have you used something that bends and springs back? A bungee cord, a mattress, a ballpoint pen clip, a car suspension, the sole of a running shoe.
Choose one. Write down what is being stretched, and what happens if it is stretched too far. You will be able to say something quantitative about it by the end of the chapter.
Sections
- 5.1 Hooke's law — force, extension and the spring constant
- 5.2 The spring constant and combinations of springs — series and parallel
- 5.3 Elastic potential energy — where the work goes when you stretch a spring
- 5.4 Investigating Hooke's law — measuring it yourself, and what the graph shows