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

Before you read this, I suggest you read post 17.15. We will think about this question by considering a plane whose wings have the cross-sectional shape shown in the picture below. In this picture, the engines of the plane move it in the direction shown by the arrow. Now let’s think about the movement of… Continue reading 17.16 Why do planes fly?

Before you read this post, I suggest you read post 17.5. A fluid is anything that flows – like a liquid or a gas (post 16.37). In post 17.5, we saw that a fluid remains stationary when there is a surrounding pressure that acts equally in all directions. However, when the pressure is greater in… Continue reading 17.15 Fluid flow

According to the Bible (in the book of Genesis, chapters 9-11), the world was once covered by water, in a great flood. A man called Noah built a big ship (called the “ark”) that he used to rescue his family and all the species of animals. After the flood, his descendants lived in a place… Continue reading 17.14 Confusion

Before you read this, I suggest you read posts 17.12, 17.11, 17.10 and 17.4. The man in the picture is practising “throwing the hammer”. In this event, the participant swings a spherical “hammer” using a cable, as shown in the picture, several times around his/her body. So the hammer goes round in a circle, with… Continue reading 17.13 Centripetal force – throwing the hammer

Before you read this, I suggest you read posts 17.4 and 17.11. In the picture above, the point P is moving in a circular path around the point O. In a time-interval, Δt, it moves from P to P’, and sweeps out an angle Δθ. (In this post the symbol Δ, the Greek letter capital… Continue reading 17.12 Going round in circles

Before you read this, it might be useful to read post 16.7. The most common way to measure angles is to divide a complete revolution (for example, of a wheel) into 360 degrees, usually written as 360o. Then a right-angle is a quarter of a revolution, 360o/4 = 90o. Another way is to divide a… Continue reading 17.11 Measuring angles

Before you read this, I suggest you read post 16.50. Let’s suppose we want to use the (grey) spanner to loosen the (red) nut, in the picture above. We apply a force, F, that makes an angle θ with the axis of the spanner at a distance L from the axis around which the nut… Continue reading 17.10 Torque – using a spanner

Have you ever read the phrase, “Scientists believe that…”, or has anyone ever said it to you? What does it mean? I think it means that something is supposed to be too difficult for you to understand but somebody in authority believes it – so you ought to believe it too! Fortunately, science doesn’t work… Continue reading 17.9 Scientists believe that…