Before you read this, I suggest you read post 20.19. Torsion is the deformation you apply when you start to remove a screw-top from a bottle. In the picture above, torsion is applied to a cylinder, with a fixed base, so that A moves to A’. O is the centre of the top face of… Continue reading 21.21 Torsion

Before you read this, I suggest you read posts 21.17 and 21.19. This post is a continuation of post 21.19. I am going to use the ideas in post 21.19 to develop the equations stated, without proof, in post 21.17, that allow us to calculate the deflection and maximum stress in two examples of beams:… Continue reading 21.20 More about beams

Before you read this, I suggest you read post 21.17. In post 21.17, we saw that beams can support a load because they are bent by it. I also gave some rules for calculating the deflection of some simple beams and the maximum stress in them. But I didn’t explain how these rules are derived.… Continue reading 21.19 Bending a beam

In post 20.15, we saw how to calculate the forces that could maintain a system in equilibrium. We developed equations, by equating the vector sum of the forces acting on the system and the vector sum of the torques acting on the system, that could be solved to find the forces. But suppose the equations… Continue reading 21.18 Statically indeterminate systems

A door, or a window, fills a hole in a wall. The wall above the hole is supported by a horizontal beam, called a lintel. Gravity exerts a force Mg on the mass, M, of the wall above the beam, where g is the acceleration due to gravity, as explained in post 16.16. Note that… Continue reading 21.17 Beams

This post is about equilibrium in statics – the title could cause confusion because the word “equilibrium” is used in science in several different contexts. In post 21.15 we saw that statics describes a system that is not moving in the frame of reference of an observer – it is in equilibrium. We also saw… Continue reading 21.16 Equilibrium

It is easy to confuse the words “statics” and “statistics”. Statistics is about using numbers to test ideas and make predictions. Statics is about the forces that are acting on things that are not moving. (The study of forces acting on moving objects is called dynamics.) My simple definition of statics is easy to understand… Continue reading 21.15 Statics

Before you read this, I suggest you read post 21.13. When Crick and Watson proposed that the molecular structure of DNA was a double helix, it immediately suggested to them a mechanism for how genetic information could be copied during cell division in mitosis and meiosis. This mechanism was based on the idea that genes… Continue reading 21.14 Copying genetic information

To make an object go round in a circle, we must exert a force that pulls it towards the centre of the circle. This force is called the centripetal force. If the object is a container, any object within the container experiences a force of F = mω2r                     (1) that pushes it away from the… Continue reading 21.13 Centrifugation

Before you read this, I suggest you read post 21.2. Most people who are interested in science know that the DNA molecule is a double helix. But what does this mean? How do we know? And why is it important? By the early 1950s, there was evidence that genes were made of DNA and that… Continue reading 21.12 The DNA double helix