You cannot be serious
According to basic aerodynamic teaching, an aerofoil wing generates lift because air passes over the upper surface faster than below the undersurface. A tennis ball that has been given topspin also has air passing over the top of it faster than underneath it, but the ball will be pulled down not up. This would seem to contradict the aerofoil rule. What is happening?
* The classic description of lift, mentioned in the question, is an incomplete description that clearly fails in the case of the spinning ball. The way to understand the spinning tennis ball is to approach the question through Newton鈥檚 laws. From Newton鈥檚 third law, lift is a reaction force that is created because a wing diverts air down. The quick answer to the tennis ball question is that topspin diverts air upwards, and therefore the ball experiences a downward force.
鈥溾�漈opspin diverts the air upwards, and therefore the ball experiences a downward force鈥濃��
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With topspin, the velocity of the air relative to the surface at the top of the ball is higher than that relative to the bottom. Because the airflow cannot smoothly negotiate the back surface of the ball, it separates, forming a wake. The point at which separation occurs on the top is forward of where it occurs on the bottom. This gives an asymmetric wake with the wake bent slightly up. The ball reacts and feels a net force down.
A ball without spin has a symmetric wake, and no net force up or down. A ball can be given sidespin to induce a curved path. Under certain conditions, depending on the size of the ball and its speed, a periodic, asymmetric wake may develop. These fluctuations cause the ball to appear to corkscrew as it travels. In baseball, a ball thrown to develop this erratic behaviour is known as a 鈥渒nuckle ball鈥�.
Scott Eberhardt, Department of Aeronautics and Astronautics, University of Washington, Seattle, US
Scott Eberhardt is co-author with David Anderson of Understanding Flight (McGraw-Hill, 2001) 鈥� Ed
* The fact that fast-moving air exerts lower pressure than air moving slowly is known as Bernoulli鈥檚 principle. However, this is, at best, only a rule of thumb. The real explanation of how an aerofoil generates lift is much simpler. As air flows over an aerofoil wing it is forced downwards, which creates an upward force on the wing as a consequence of the reaction in Newton鈥檚 third laws of motion.
As a tennis ball with topspin moves forward, the trailing edge is moving upward, so it will tend to propel air upwards in its wake, which is why there is a downward force on the ball.
Andrew Page, Epsom, Surrey, UK
* Spinning objects such as balls deviate from their expected path because of the Magnus effect. If the ball has topspin, the thin layer of air in contact with the ball鈥檚 surface (the boundary layer) is actually travelling faster at the bottom than the top. This makes the airflow in the lower boundary layer stick to the ball longer than the top boundary layer. The upper layer breaks away before the lower, which leads to an upward force on the airflow with 鈥� thanks to Newton 鈥� a downward force on the ball.
The British engineer Barnes Wallis used a similar effect resulting from backspin to make his bouncing bombs travel further. For a more detailed explanation, go to
Peter Elliott, Senior Keeper, Department of Research and Information Services, Royal Air Force Museum, Hendon, London, UK
Doggy choccy
I have read that dogs should not be fed chocolate because it is poisonous to them. Is this correct, and if so, what are the differences between canine and human digestive processes? And what are the chocolate drops that you can buy for dogs really made of?
* It is true that chocolate is poisonous to dogs. The problem is that chocolate contains cocoa, which in turn contains methylxanthines. These chemicals, which act on blood vessels, include theobromine, theophylline and caffeine. They are responsible for the 鈥渒ick鈥� we get from coffee, tea and chocolate.
The reason why dogs react differently to chocolate lies in their liver enzymes. In dogs, breakdown of the methylxanthines takes a long time. For example, theobromine takes about four days to be cleared from a dog鈥檚 system. It affects the heart, central nervous system and kidneys, and even a small dose can kill a dog in as little as six hours. The amount of chocolate that proves toxic varies, depending on the amount of cocoa used in the recipe, as well as variations in individual dogs themselves.
Many fatalities have been reported following ingestion of as little as 5 grams per kilogram of bodyweight. Medical records show that two English bulldogs died after eating about 25 grams of dark chocolate each, and a springer spaniel died after eating around a kilogram of milk chocolate. Others have died after eating cooking chocolate, cocoa powder and cocoa-based garden mulch 鈥� as single doses or repeated exposure to smaller doses.
The only safe option is not to give human chocolate to dogs. Doggy chocs are available in pet shops, marketed as 鈥渃hocolate substitutes鈥� that are safe for dogs. They contain either no cocoa, or very little. Check the ingredients with care; if you would prefer to give your pet no cocoa, choose one of the many cocoa-free options. After all, dogs are carnivores and there isn鈥檛 any chocolate in the average carcass.
Kim Russell, South Molton, Devon, UK
This week鈥檚 questions
No-ball snow
On holiday in Scotland last winter I found it impossible to make snowballs with that day鈥檚 newly fallen snow. It was an extremely cold day and it would not stick together in the way it normally does. My friend has also experienced this in the Alps. What is the reason for it?
Morag Challenor, Dundee, UK
The sound of silence
When I am in an essentially silent place I can hear a very faint droning or hissing. Can you tell me what this 鈥渟ound of silence鈥� white noise is and why I hear it?
S. Lewis, Bridgton, Maine, US,
Push and pull
Why, in an atom, does the negatively charged electron not collapse into the positively charged nucleus? Is this in any way similar to reason why large systems like stars and planets do not collapse into each other under the pull of gravity?
Daniel Doe, Northamptonshire, UK