Many people believe two things about osteoporosis that are not true: It makes bones more brittle It is caused by loss of calcium from bones. Osteoporosis makes bones less strong but not more brittle. Osteoporotic bone breaks at a lower stress than normal bone – so it’s less strong. But it does not appear to… Continue reading 20.14 Osteoporosis 1 – bone structure

Before you read this, I suggest you read posts 20.9 and 20.10. In post 20.10 I described toughness as the energy absorbed by a unit volume of material before it fractures. It is often simpler to understand the process of fracture (when something breaks, post 20.10) in terms of energy rather than the stress (or… Continue reading 20.13 Fracture

Like most people, I hadn’t thought much about this topic until the last few weeks. But covid-19 means that every day we see numbers related to the spread of a pandemic. An epidemic has been defined as the “widespread occurrence of infection in a community in a given time”. A pandemic has been defined as… Continue reading 20.12 What happens in a pandemic?

Before you read this, I suggest you read posts 20.2, 20.6 and 20.8. In post 20.5, we saw that a material that maintained a constant volume, when it is stretched, has a Poisson’s ratio of 0.5. The Poisson’s ratio of rubber is approximately equal of 0.5, so any volume change, when it is stretched, are… Continue reading 20.11 Elasticity of rubber

Before you read this, I suggest you read post 20.9. In everyday English “toughness” and “strength” are used to mean the same thing. But in science such words are often given very specific meanings to avoid confusion (post 17.14). Then “toughness” and “strength” have precise and different meanings. It is sometimes helpful to have an… Continue reading 20.10 Toughness

Before you read this, I suggest you read post 20.2. Concrete is stiff – you can’t see it change dimensions or shape (it doesn’t deform very much) when subjected to a force. Steel is stiff too – you don’t usually see appreciable deformations in most of its applications. We think that concrete and steel are… Continue reading 20.9 Stiffness and strength

Before you read this, I suggest you read post 20.7. Natural rubber is a polymer consisting of isoprene molecules joined as shown in the picture above. It is obtained from the sap of rubber trees which consists of the polymer molecules dispersed in water. The water is removed and the polymer rolled into thin sheets.… Continue reading 20.8 Rubber

Before you read this, I suggest you read posts 16.30 and 16.33. At high pressure, in the presence of a suitable catalyst (post 16.33) ethene molecules (see picture above) join to form polyethene or polyethylene or polythene, whose formula is shown below. Notice that each carbon atom is still bonded to two hydrogen atoms, as… Continue reading 20.7 Polymers

In posts 20.2, 20.3 and 20.5, we thought about stretching a long, thin wire. This post continues the topic. But it is about the work done (post 16.20) in stretching the wire and what happens to the resulting potential energy (post 16.21). When a force stretches the wire, it moves its point of application and… Continue reading 20.6 Elasticity

Before you read this, I suggest you read post 20.2. Let’s return to stretching the long, thin wire of post 20.2. In posts 20.2 and 20.3, we neglected that the wire becomes slightly thinner as it gets longer. The reason for thinking about a long, thin wire was that this effect is not easy to… Continue reading 20.5 Poisson’s ratio