We have seen that planes fly because air provides “lift” under their wings (post 17.16); any engines that moves the plane forward will enable air to lift the plane. But there is no air in space (because air molecules are attracted to earth by its gravitational field – see post 16.16) so space-craft need… Continue reading 17.26 Rockets

Acceleration is a simple idea – suppose a car increases its speed from 0 to 60 km.h-1 in 3 s. Its final speed is 60/(60 × 60) km.s-1, so its average acceleration is 60/(60 × 60 × 3) = 0.006 km.s-2 (see post 16.13 for more explanation, and post 16.7 if you’re worried because the… Continue reading 17.25 Diminishing deception

Before you read this post, I suggest that you read posts 16.50, 16.25 and 16.16. In posts 16.16 and 16.25, we met the idea of a field – a region of space in which an object experiences a force that depends on its properties. For example, an object experiences a force in a gravitational field… Continue reading 17.24 Fields and vectors

Before you read this, I suggest you read posts 17.19 and 17.22. This post uses the mathematical technique of integration (post 17.19) to explore further the concepts of centre of mass (post 17.21) and centre of gravity (post 17.22). If you don’t like mathematics, you probably won’t want to read this post. In post 17.22,… Continue reading 17.23 Integration, centre of gravity and centre of mass

Before you read this, I suggest you read posts 16.16, 17.10 and 17.21. We expect a rod, whose cross-section and density remain constant along its length, to balance at its centre. Why? The picture above shows a rod balanced in the way we expect. We could consider the rod to be made up of a… Continue reading 17.22 Balancing

If you have been reading and thinking about this blog regularly, you may have noticed that I often ignore the dimensions of an object. For example, when we calculated the speed that a falling object hits the ground (post 17.18), we assumed that the object was a point in space. In this example, assuming that… Continue reading 17.21 Centre of mass

Before you read this, I suggest you read post 17.19. This post illustrates the application of integration to calculate velocity and speed from acceleration, as described in post 17.19. In this post, we will consider an object (like the rock in the picture above) falling close to the surface of the earth, so that the… Continue reading 17.20 Falling

Before you read this post, I suggest you read post 17.4. This post introduces the mathematical concept of integration that makes it much easier to understand many scientific topics. But, if you really don’t like mathematics, you can ignore this post and still understand most parts of most other posts. To keep it simple, I’m… Continue reading 17.19 Calculating distances from speeds – integration

Before you read this, I suggest you read post 17.17. If we ignore viscosity (post 17.17), an object of mass m that falls from a height h, gains a speed of v =√(2gh) (post 16.42), where g is the magnitude of the earth’s gravitational field (post 16.16). This result was derived by assuming all the… Continue reading 17.18 Terminal velocity and parachutes

Try pushing your finger slowly through a glass of water: now try to do the same in a glass of treacle or honey. You will find that the treacle or honey exerts a force (post 16.13), resisting the motion of your finger (rather like friction in post 16.19), that is much greater than the force… Continue reading 17.17 Drag and viscosity