Chemical reactions: blow hot, blow cold

Life seems to thrive on the fact that warm and cold are, well..., two different temperature ranges! It is November now, and the northern hemisphere heads into winter just as the southern hemisphere starts heating up to start up their summers.

This magical arrangement is caused by the Earth's tilt, the same feature that creates the weather (winds, rain, storms, clouds, precipitation), and the seasons, and many things that are interesting about the Earth's surface.

As if to mirror this property of Nature, every known chemical reaction is either endothermic  (requiring, or drawing in heat) or exothermic (giving off heat). An easy way to remember which is which, is to remember that "endo" is Latin for within and sounds like "indo" - heat is pulled in. Exo is Latin for outside, so it describes reactions that give off heat, sending heat outside. Simply put, an exothermic reaction gives off heat and feels warm.

When two or more chemicals react, it is really their molecules that come up against each other and react. Some molecules have high valencies (available, unpaired electrons) and so react easily at room temperature, usually releasing heat energy when the chemical bonds are broken. Other molecules need to be heated to reach a level of energy at which old chemical bonds can be broken to form new ones. The former are exothermic, and the latter are endothermic. Chemical reactions that occur in nature tend not to be thought of as "chemical reactions" because there is no laboratory in sight, but they are valid chemical reactions all the same - the formation of ammonia in a lightning storm, the rusting of iron, the fermentation of barley and hops to form beer, and so on.

Rusting iron gives off heat, whereas the fermentation in baking bread requires heat. Making ice cubes gives off heat (surprising? But unless a body of water lost more heat than it absorbed, it could not get colder, and turn to ice) whereas melting ice absorbs heat and so is endothermic.

Can you think of other commonly seen reactions, and classify them as endothermic or exothermic?

iPad apps for Physics: A great Thanksgiving gift idea!

The Exploriments blog wishes all its readers a Very Happy Thanksgiving!

Greetings to those who celebrate Thanksgiving by enjoying the long weekend with friends and family and the science geeks who'd rather curl up with a good science blog or video - spare us a few moments now, and you'll thank us later when "Back to School" madness begins.

We have made Physics fun for middle and high school students by bringing our apps to the Apple Store and iTunes. Our interactive learning objects continue to be available online at, but now they're also at your fingertips, on your favorite device the Apple iPad. We at Exploriments know the benefits of simulation-based learning, and we would love to get the good word out to everyone in the community.

If you are a 1:1 iPad school, a committed parent or a passionate teacher, Exploriments will be your best friend. As of this writing, there are Exploriments for iPad available in the areas of Motion, Fluids, Electrostatics, Electricity, Force and Light - all using interactive touch-based simulations, with illuminating content as well as recommended explorations. 

View Exploriments on iPad now and bookmark the link too - we update the tabs as we build and release new apps. Try the free Weight and Mass app to get a taste of simulation-based learning - it is fun and easy to understand. Happy Exploring! 

Sandy and Nilam. Those depressing visitors!

Hurricane Sandy began receding on October 31 2012 from the Eastern seaboard of the United States, and cyclone Nilam hit the Eastern seaboard of India on November 1 2012. Both are examples of naturally recurring phenomena that create havoc and cause losses worth billions of dollars, and hundreds of lives every year. The affected areas are still limping back to normalcy.

Cyclones ("hurricanes" in North America and "typhoons" in Japan) are giant storms that have high winds and heavy rainfall produced by a circling vortex of clouds that can be over a thousand miles across. Cyclones are caused by weather depressions. But they are also the cause of fiscal and mental depression because of the damage and chaos they cause.

You can better understand low pressure or depression by understanding pressure itself. 

The Earth is wrapped in a 75-mile-thick film of air; to scale, that is like the film of water on a wet basketball. Towering over any point on the Earth's surface is a column of air approximately 75 miles high. This column of air, with help from the Earth's gravitational pull, exerts a force. Force measured per unit area is called "pressure", and the air we breathe exerts a measurable atmospheric pressure on everything around us, at all times. 

The air is not equally dense at all altitudes - 75% of the air is available closest to the Earth, a zone going all around that is less than 11 miles high. This is the air made up mainly of Nitrogen and Oxygen that all creatures breathe. As you go higher, the density of air reduces as there are fewer and fewer molecules of air available, and it gets more and more difficult to breathe. On the surface of the Earth, this pressure - commonly known as atmospheric pressure and measuring 1 bar or 101,315 Pascals - is equivalent to a standing column of 76 cm of Mercury. In other words, the pressure exerted by 75 miles of atmospheric air equals the pressure exerted by 76 cm of the much denser fluid, Mercury.

An atmospheric depression, therefore, is a low pressure zone within the Earth's atmosphere. It is caused because differing sunshine from the tropics to the poles causes differing ocean temperatures and air temperatures, which in turn causes low and high pressure regions. The characteristic of a low pressure region is that air from surrounding areas rushes in to equalize the pressure. Put very simply, air rushing in to fill an atomospheric depression is what produces high winds, that sometimes go on to create a hurricane; the spinning, moving vortex is formed because the zone of depression itself keeps moving due to the rotation of the Earth.
The greater the difference in ocean temperature, the greater the depression, the greater the velocity of winds, and the greater the storms that are produced. Ever since the invention of barometers, it has been noticed that a drop in air pressure could mean the onset of a storm. Climate change in recent decades has meant higher and higher temperatures, which produce deeper depressions and bigger, more destructive storms.

Keep in mind that 1 bar, or 1000 millibar, is the normal atmospheric pressure at sea level. The atmospheric pressure at landfall of hurricane Sandy (940 millibar) was lower than the pressure of cyclone Nilam (990 millibar). The apparently small difference in pressure, however creates a much magnified effect at the scale of a storm - Sandy had winds as high as 110 mph compared to the 50 mph winds of cyclone Nilam. You can track global temperatures, see how they affect barometric pressures, and understand how depressions form and produce storm systems.

Fearless Felix's free fall, and a discussion of terminal velocity.

14 October 2012: In a much-televised event Felix Baumgartner jumped off a pod floating 39 km (24 miles, or 128,000 feet) over the Earth's surface and straight into the history books.

In one fell swoop, he set the record for the highest jump and the highest speed ever achieved by a non-powered human being in air - 1.24 Mach. Yes, quite astoundingly, he reached a speed of 1,342 kilometers per hour (834 mph), which is 1.24 times faster than the speed of sound. Despite his preparation and the modern technology available to him, an earlier jump by his mentor Joseph Kittinger remains the record for the longest free fall (4 minutes, 36 seconds!)

Felix Baumgartner at work:

Kittinger's free fall record, set way back in 1960,  reached a speed of 988 kilometers per hour (614 mph). While Kittinger's is truly remarkable for being an outrageously bold pioneering attempt, Baumgartner's is special for breaking the sound barrier.

A force "F" that moves a body of mass "m" through a fluid, does so with a resultant acceleration "a", since F = ma. Acceleration causes velocity to steadily increase. Since any real fluid is not without resistance, movement happens at the cost of the body overcoming the fluid's resistance. Eventually, there is a point at which the motive force equals fluid resistance. At this point, the resultant force on the body is zero, which means its acceleration goes to zero, and it cannot go faster than the velocity it has achieved, the so-called "terminal velocity". Any body freely falling through the Earth's atmosphere accelerates at the rate of 9.8 m/s, gradually increases its speed until air resistance nullifies the acceleration.

These achievements, entirely credit-worthy though they are, can create a small doubt in the minds of science aficionados in the matter of terminal velocity: How did these gentlemen manage to exceed the terminal velocity, which is known to be 195 kilometers per hour (122 mph or 54 m/s)?

The answer, my friend is blowing in the wind; terminal velocity is a function of air resistance, and that presupposes the existence of air. Most of the air molecules in the Earth's atmosphere exist below an altitude of 5.5 km. The altitude from which this jump was executed is seriously lacking in air molecules, and therefore lacking in air resistance. Without air resistance, there is theoretically no upper limit to the velocity achievable by a body under acceleration, and that is how Felix Baumgartner was easily able to surpass the terminal velocity as well as the sound barrier.

Assuming that for most of the fall Felix Baumgartner encountered only negligible air resistance, the distance he would have fallen to reach the speed of sound (with initial velocity u = 0, final velocity v = 340 m/s and acceleration = 9.8 m/s):

d = (v2 - u2)/2g = 3402/2x9.8 = 115600/2x9.8 = 5898 m or 5.8 km

Also, distance in terms of time taken "t" and acceleration "g", where initial velocity is zero, is:
d = gt2/2

Therefore, the time taken to reach this velocity would have been:
t = sqrt(2d/g) = sqrt(2 x 5898/9.8) = 34.69 seconds.

Comparing the zero-air-resistance calculations to the facts of his free fall (39 km in about 260 seconds), you get an idea of what a great deterrent air resistance is - even when much reduced.

Sports Science: The Unpredictable Knuckleball

What do all sports have in common? Activities as diverse as baseball, basketball, cricket, swimming, gymnastics, snooker/billiards, carrom (or carroms) and cycling all involve an application of one or more principles of science. Sports, with its preoccupation with balance, trajectory, speed, timing and spin, is nothing but a living and breathing science laboratory.

What better way of starting a sports segment on the Exploriments blog than with the national US pastime - baseball? Baseball is so fascinating because it is a duel between the batter's skills and the pitcher's skills. While pitchers frequently employ the fastball, it is the knuckleball that is a more interesting Physics study.

The main skill in throwing a knuckleball, other than being able to dig your fingernails in to hold the ball, is to not impart any spin to it. A good knuckleball turns only a few times during its entire flight. The fact that it has no spin leaves it susceptible to wind eddies and the vortices that form over its seam, and this makes it float in unpredictable ways. 

A regular fastball spins fast about its own axis, giving it the angular momentum and rotational inertia which helps it resist changes to its position. This gyroscopic effect helps the ball retain its orientation and course, making it more predictable to follow around and to catch or hit. A knuckleball neither has the spin, nor the gyroscopic effect associated with spin, making the trajectory of the ball difficult to pick out. This video from Reuters TV shows this Physics principle in action: