Want to know how fast your car's going? It's easy — just glance at thespeedometer!
Speed is a handy measurement that tells you how quickly you can
get from one place to another. A car's top speed is generally a good indication of how powerful an engine it has, but assuming everyone stays within the speed limit, maximum speeds are just numbers on paper—little or no use to anyone.Acceleration is much more interesting than speed and more useful if you need to get out of danger behind the wheel: it's how quickly something can speed up or slow down. Measuring acceleration is a bit more tricky than measuring speed because it involves figuring out how speed changes over a period of time. How do you measure acceleration? Not surprisingly, with a device called an accelerometer.
Once upon a time you'd have found gadgets like this only in space rockets or jumbo jets; now they're in virtually every car, many laptop computers, and all kinds of gadgets such as iPods, iPhones, and the Nintendo Wii. Let's take a closer look at what they are, what they do, and how they work!
What are accelerometers used for?
Accelerometers are the stuff of rocket science—quite literally! ounted in spacecraft, they're a handy way to measure not just
changes in rocket speed but also apogee (when a craft is at its maximum distance from earth or another mass, so its acceleration due to gravity is at a minimum) and orientation (because tilting something changes the way gravity acts on it and the force it feels). Accelerometers are also widely used in inertial navigation and guidance system in such things as airplane and ship autopilots. Another very common use in transportation is in automobileairbags: when anaccelerometer detects a sudden change in a car's speed, signaling an imminent collision, it triggers an electrical circuit that makes the airbags inflate.If you have a modern cellphone, MP3 player, or handheld games console, it probably has an accelerometer built into it so it can sense when you tilt it from side to side. That's how an iPhone or an iPod Touch automatically figures out when to switch the screen layout from portrait to landscape. Many games and "apps" designed for gadgets such as iPhones work by sensing how hard or how fast you move or shake the case using tiny accelerometer chips inside.
You'd be surprised at some of the things
accelerometers are used for. Did you know, for example, that hi-tech clothes washing machines have accelerometers that can detect when the load is out of balance and switch off the electric motor to stop them from spinning themselves to pieces? Or that heating appliances, such as electronic irons and fan heaters, have accelerometers inside that detect when they fall over and switch them off to stop them causing fires? Amazing, eh? Isn't rocket science useful!
What is acceleration?
Before you can understand accelerometers, you really need to understand acceleration—so let's have a quick recap. If you have a car that accelerates from a standstill to a speed (or, strictly speaking, velocity) of 100km/h in 5 seconds, the acceleration is the change in velocity or speed divided by the time—so 100/5 or 20 km/h per second. In other words, each second the car is driving, it adds another 20km/h to its speed. If you're sitting inside this car, you could measure the acceleration using a stopwatch and the car's speedometer. Simply read the speedometer after 5 seconds, divide the reading by 5, and you get the acceleration.
But what if you want to know the acceleration moment by moment, without waiting for a certain time to elapse? If you know about the laws of motion, you'll know that the brilliant English scientist Isaac Newton defined acceleration in a different way by relating it to mass and force. If you have a certain force (say, the power in your leg as you kick it outward) and you apply it to a mass (a soccer ball), you'll make the mass accelerate—the ball will shoot off into the air.
Newton's second law of motion relates force, mass, and acceleration through this very simple equation:
Force = mass x acceleration
F = m a
a = F / m
In other words, acceleration is the amount of force we need to move each unit of mass. Looking at this equation, you can see why soccer balls work the way they do: the harder you kick (the more the force), or the lighter the ball (the less the mass), the more acceleration you'll produce—and the faster the ball will fly through the sky.
You can also see we now have a second way of calculating acceleration that doesn't involve distance, speed, or time. If we can measure the force that's acting on something and also its mass, we can figure out its acceleration simply by dividing the force by the mass. No need to measure speed or time at all!