Most people have seen Einstein’s famous equation E = mc2.                    (1) But what does it mean? It means that if stuff with a mass m is completely destroyed, it changes into energy E, when c represents the speed of light in a vacuum. When I say, “completely destroyed”, I don’t broken or burnt (burning simply… Continue reading 17.41 Units in equations

Before you read this, I suggest you read posts 17.38 and 17.39. We are all familiar with rolling objects (a ball rolling along the ground or the wheel of a moving car); as a result, we tend not to think much about rolling and may not appreciate how complicated it is. The picture above shows… Continue reading 17.40 Rolling

In posts 17.11 and 17.12 we saw that angular displacement, angular speed and angular acceleration can be defined, as vectors, in a way that is analogous to displacement, velocity and acceleration (post 17.4) along a line (not necessarily a straight line) in space – translational motion. In post 16.12, we saw that translational motion could… Continue reading 17.39 Translational and rotational motion

Before you read this post, I suggest that you read posts 17.12 and 17.19. Let’s think about a particle (an object of negligible size) of mass m moving around a circle of radius r with a constant angular speed ω (see post 17.12). Its speed is given by v = ωr (see post 17.12) so… Continue reading 17.38 Rotational inertia

Before you read this, I suggest you read posts 17.2, 17.3 and 17.10. In post 17.25, we saw that it is often better to introduce an idea as simply as possible and to add further sophistication at a later stage. For example, in post 16.20, the idea of work was explained by the work done… Continue reading 17.37 More about torque – cross products of vectors

Before you read this, I suggest you read posts 16.20, 17.4 and post 17.19. But if you want to stop at the seventh paragraph, there is no need to read 17.19. In post 16.20, we saw that a force does work when it moves something. This is a simple definition of work and there is… Continue reading 17.36 More about work – line integrals

Before you read this, I suggest you read posts 16.19 and 16.50. The picture above shows the forces acting on an object on a slope – like the vehicle on a road camber in post 17.34. If the object has a mass of m, gravity pulls it towards the centre of the earth with a… Continue reading 17.35 Sliding

Before you read this post, I suggest you read posts 17.13, 17.16 and 17.17. The picture above shows a skier making a right turn by leaning to the right and transferring his/her weight on to the right-hand ski. The pictures above show a motorcyclist turning right by leaning right and a plane turning left by… Continue reading 17.34 Turning corners

William Thomson was one of the most famous physicists of the 19th century. He was born in 1824 and graduated in mathematics, from the University of Cambridge, in 1845. By 1846 he was Professor of Natural Philosophy (the contemporary name for physics) at the University of Glasgow in Scotland. He was made Sir William Thomson… Continue reading 17.33 Lord Kelvin and flying machines

Before you read this, I suggest you read posts 17.16 and 17.31. We often see birds flying without flapping their wings – they are gliding (post 17.31). In order to glide, their wing cross-sections must be shaped so that air moving over the wing has further to travel than air moving under the wing, so… Continue reading 17.32 How do birds fly?