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Shaking up water heats it, Mammals will eat anything

Q: We all know that friction produces heat. That's how prehistoric men made their fire. If you put water in a bottle and keep shaking it, does the water temperature go up? (Naguib)

A: Yes it does. Temperature indicates nothing more than the average movement (kinetic energy) of the water molecules relative to each other. The faster the molecular motion, the higher the temperature. Shaking the bottle causes the water molecules to move faster and therefore the temperature to rise.

Of course, you need to shake the bottle a long, long time to raise the temperature enough to notice by touch. The amount of heat needed to raise a pound of water (0.45 kg) by one Fahrenheit degree is equivalent to raising the water 778 feet (237 m) — about as high as the Eiffel Tower in Paris, 80 stories up.

James P. Joule was the first to demonstrate that shaking water heats it. [Public domain, Wikipedia]

In 1845, James P. Joule first demonstrated this phenomenon in his "Mechanical Equivalent of Heat" experiment.

Describing his results, he said he filled a "can of peculiar construction" with water and placed a "brass paddle-wheel" into the container.

He then wound a cord around the top of the paddlewheel and released weights (a known force). The dropping weights, connected to the paddlewheel by the cord through pulleys, caused the paddles to churn the water — like an old-fashioned ice cream maker stirring its custard. In other words, he shook the can.

Joule’s “peculiarly constructed can” and brass paddlewheel (illustrated suspended above the can to show how it fits in). [University of Aberdeen, Natural Philosophy Collection]

He measured the distance that the weights moved

and thus knew how much work (work = force x distance) the paddles did.

Using "a very sensible and accurate thermometer", he measured the temperature increase. "The force spent in revolving the paddle-wheel produced a certain increment in the temperature of the water," he concluded.

Shaking water reveals much:

Further Reading:

Hyperphysics by Rod Nave: Temperature

Hyperphysics by Rod Nave: First law of thermodynamics

Q: What do most mammals eat? I've heard that 5% of mammals are carnivores, which makes me wonder... Are most mammals herbivores? Are frugivores relatively rare and insectivores quite common? Are there any blood eaters other than the vampire bat? (Dia, Washington DC)

A: Most animals (roughly 85%) eat plants or their products (such as seeds, fruit, and nectar). Plants are the most readily available food.

Animals eat to get energy for reproduction and to maintain life. The Sun provides Earth’s energy. Only plants convert the Sun’s energy directly into sugar and starches — the chemical energy that feeds life. Furthermore, that energy is cheaply available since its raw materials (sunlight, carbon dioxide, and water) are plentiful.

So, animals must eat plants or plant-eating animals to get food energy. Plants are abundant and, therefore, most animals have developed traits (like a cow’s three stomachs) that help them eat and digest plants in quantity. Actually, almost all animals eat plants to some extent — even carnivores.

Plants, however, are not energy-rich food, says Ron Brooks, zoology professor at the University of Guelph in Ontario. Cows eat vast quantities to get enough energy. Plants are just cheap. Meat is energy rich but dear since the Sun’s energy travels a longer, more inefficient route to become food. Plants efficiently convert Sun’s energy to food. Plant eaters inefficiently convert food energy into their bodies — meat (to meat eaters).

"This is especially true for mammals because of their high metabolic costs," says Brooks. "So, carnivores make up a much smaller biomass than their herbivore prey and the herbivores much less than their plant prey."

Which brings us to your question about carnivores. The carnivore order of mammals includes 252 species out of the 4,237 mammalian species — or about 6%. Not all these species eat only meat, however. Raccoons, civets, jackals, badgers, skunks, and bears also eat fruit, honey, seeds, roots, and other plant foods. The panda (a carnivore) eats bamboo shoots (plants) almost exclusively.

The lynx hunts at night— pouncing on chipmunks, lemmings, rats, and an occasional small roe deer. [Erwin and Peggy Bauer, US Fish and Wildlife Service]

Pandas are carnivores because they belong to a set of animals with a common ancestry — true bears. But not all bears (such as the panda) exhibit features (like eating meat) that characterize most of the group. In other words, not all Carnivora are carnivorous.

Many herbivores (such as mice and rats) also eat meat. In fact, some mice live in meat storage lockers, where the temperature is below freezing, and eat only meat. So, grouping mammals by what they eat puts mammals in more than one group. At best, diet-based percentages are approximate. Animals, eating to survive, aren’t picky.

Yes, the number of frugivores (exclusive fruit eaters, like flying squirrels) is few (374 species, 9% of all mammals) compared with insectivores. Many animals do eat fruit when they find it, though. Deer scarf down apples in the fall.

"Insectivores are fairly common because insects are very common but the small size and low nutrient value of insects means that most insectivorous mammals are small, except the anteaters." Anteaters eat termites that live in large colonies, so pickings are plentiful and anteaters grow big.

And finally, no, there is no other mammal besides the vampire bat (Desmodus rotundus) that lives strictly on a diet of blood, though many animals like blood.

Further Reading:

University of Guelph, Zoology: Large Herbivores by Ron Brooks

Morris, Desmond, The Mammals, a guide to the Living Species, New York: Harper & Row, 1965.

(Answered April 22, 2005)