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
If we believe that chromosomes contain genes (see post 21.10), then genes must be made of either DNA or a type of protein called a histone – because that’s what chromosomes are made of. The question is then – are genes made of DNA or histones? In the early 1940s, most people believed that genes… Continue reading 21.11 Genes are made of DNA
Before you read this, I suggest you read post 21.9. In post 21.9, we saw how cells can divide to produce copies of themselves during growth or to replace old cells in a process called mitosis. During mitosis, chromosomes appeared in the cell nucleus. These chromosomes appeared in pairs, so we can say that each… Continue reading 21.10 Chromosomes 2 – meiosis
A biological cell is transparent and so is difficult to see with a microscope. Biologists usually dye cells before they use a microscope to examine them. The dyes they use are called stains and the dyeing process is called staining. A stained animal cell then shows the features in the picture above – it is… Continue reading 21.9 Chromosomes 1 – mitosis
