Answers to last issue's Do You Know?

1. How do gears (in cars and some cycles) work?

Ans: Gears are helpful in machines of all kinds, not just cars and cycles.  They are a simple way to generate more speed or power or send the power of a machine off in another direction.
A car engine makes power in a fairly violent way by harnessing the energy locked in gasoline. It works efficiently only when the pistons in the cylinders are pumping up and down at high speeds--about 10-20 times a second (or 600-1200 times a minute). Even when the car is simply idling by the roadside, the pistons still need to push up and down roughly 1000 times a minute or the engine will cut out.
In other words, the engine has a minimum speed at which it works best of about 1000 revolutions per minute (1000 rpm). But that creates an immediate problem because if the engine were connected directly to the wheels, they'd have a minimum speed of 1000 rpm as well--which corresponds to roughly 120 km/hour. So, if you switched on the ignition in such a car, your wheels would instantly turn at 120 km/hour!
Suppose you put your foot down until the pistons reached 7000 rpm.  Now the wheels should be turning round about seven times faster and you'd be going at 840 km/h or as fast as a jet plane!!
It sounds wildly exciting, but there's a snag. It takes a massive amount of force to get a car moving from a standstill and an engine that tries to go at top speed, right from the word go, won't generate enough force to do it. That's why cars need gearboxes.
You can have any number of gears connected together and you can make them in various different shapes and sizes. Each time you pass power from one gear wheel to another, you can do one of three things:
Increase speed: If you connect two gears together and the first one has more teeth than the second one (generally that means it's a bigger-sized wheel), the second one has to turn round much faster to keep up. So this arrangement means the second wheel turns faster than the first one but with less force. Try it out with a different wheels of a spirograph.
Increase force: If the second wheel in a pair of gears has fewer teeth than the first one (that is, if it's a smaller wheel), it turns slower than the first one but with more force.
Change direction: When two gears mesh together, the second one always turns in the opposite direction. So if the first one turns clockwise, the second one must turn counterclockwise. You can also use specially shaped gears to make the power of a machine turn through an angle.
In the beginning, a car needs a huge amount of force and very little speed to get it moving, so the driver uses a low gear. In effect, the gearbox is reducing the speed of the engine greatly but increasing its force in the same proportion to overcome the inertia to get the car moving.
Once the car is going, the driver switches to a higher gear. More of the engine's power switches to making speed--and the car goes faster.
Changing gears is about using the engine's power in different ways to match changing driving conditions.  Whenever you gain something from a gear you must lose something else at the same time to make up for it.  The driver uses the gearshift to make the engine generate more force or more speed depending on whether hill-climbing power, acceleration from a standstill, or pure speed is needed.
What do bicycle gears do? Unlike in a car, the gears on a bicycle don't link by meshing together directly. Instead, a lubricated chain connects together the gears (sprockets) on the pedal with those on the back wheel. That's simply because the pedal and the back wheel are some distance apart and a chain is the easiest way to link them together. The picture shows the freewheel attached to the back wheel of a cycle and a number of gears on it.
The gears take power from the pedals to the back wheel. It lets you change the distance that the bike moves forward with each pedal stroke. With each complete pedal stroke, an un-geared cycle moves forward by an amount equal to the wheel circumference. At the highest gear, the rear wheel can move forward by upto four times that distance, while in the lowest gear, the cycle moves less than that distance. In low gear, therefore, the movement is less but the force is much larger so you can cycle up a very steep hill easily (but very slowly)!

2. How do cell phones work?
Ans: What makes a cell phone different from a regular phone?
To start with, one of the most interesting things about a cell phone is that it is actually a radio -- an extremely sophisticated radio, but a radio nonetheless. The telephone was invented by Alexander Graham Bell in 1876, and wireless communication can trace its roots to the invention of the radio by Nikolai Tesla.  It was only natural to combine these these two great technologies. 
In the dark ages before cell phones, people who really needed mobile-communications ability installed radio telephones in their cars.  In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower.
This central antenna meant that the phone in your car needed a powerful transmitter -- big enough to transmit about 70 km. It also meant that not many people could use radio telephones -- there just were not enough channels.
The genius of the cellular system is the division of a city into small cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid.
Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.
Each cell has a base station that consists of a tower and a small building containing the radio equipment.
The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user.
Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region.
All cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider.
When you first power up the phone, it listens for a System Identification Code (SID) -- a unique 5-digit number that is assigned to each carrier -- on the control channel.
The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message.
When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system.
Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of your phone's location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.
The MTSO communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. Now, you are talking by two-way radio to a friend.
As you move toward the edge of your cell, your cell's base station notes that your signal strength is decreasing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) sees your phone's signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell.
As you travel, the signal is passed from cell to cell.
If the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds.

3. How does a pressure cooker work?
Ans: Fill a pan with water, put it on a hot stove, and wait for the water to boil. Now you probably think the water will boil "when it's hot enough" -- and that's true, but only half true. The water will actually boil when the molecules it contains have enough energy to escape from the liquid and form water vapour (steam) above it. The hotter the water is, the more energetic the molecules are and the more easily they can escape. So temperature plays an important part in making things boil.
But pressure is important too. Pressure is the way a force acts over a surface. If you pump air into a bicycle tyre, the gas rushes about inside, colliding with the tyre walls and pressing outward, inflating the tyre.
In physics, we say the pressure on a surface is the force pressing on it divided by the area over which the force acts: Pressure = Force/Area.
The higher the pressure of the air above the pan of water, the harder it is for the molecules to break free; the lower the pressure, the easier it is. For example, water boils at a lower temperature on mountain tops where the air pressure is low.
So, suppose we could arrange things so that the air above the saucepan was actually at a much higher pressure than usual. That would make the water boil at a significantly hotter temperature, which would make cooking (for examples, potatoes, rice or vegetables) faster.
This is the basic idea behind pressure cookers.
When you fill the pan with water and place it on the stove, the water heats up and some of its molecules escape to form steam up above it.  With a normal pan, the steam would just drift off into your kitchen and disappear. But with a pressure cooker, the gasket and lid stop the steam escaping so the pressure soon builds up.
Although the water inside the pan boils, the higher pressure means it boils at a higher temperature than normal. That cooks your food more quickly. A special valve on the top of the lid allows a small amount of steam to escape, keeping the pressure higher than normal but not so high that the cooker explodes. If the pressure inside the pan builds up too much, the valve pops right out (the cooker "whistles"), rapidly lowering the pressure to a safe level again.

4. Which animal makes the loudest sounds?
Ans: Humpback whales create the loudest sound of any living creature.

5. What are the oldest living things on Earth?
Ans: Trees live much longer than any other type of plant or animal. In fact it's possible to know the age of a tree by counting the rings in its trunk. One ring generally equates to one year.
A Great Basin Bristlecone Pine tree (Pinus longaeva) called Prometheus was measured by ring count at 4,862 years old when it was felled in 1964 (see picture).
This is the greatest verified age for any individual living organism.  Another Great Basin Bristlecone Pine, known as Methuselah, measured by ring count of sample cores is, at 4,838 years old, the oldest known tree in North America, and the oldest known living individual tree in the world.