Science with a Smartphone: Accelerometer
Have you ever played a video game with a controller that used motion controls? Do you ever wonder how sometimes your phone seems to “know” that you’re moving? How do these electronic devices measure your motion? Try this activity to find out!
You’re probably familiar with the units we use to measure distance and velocity. In the US, we might say someone is 5 feet 11 inches tall (distance), or that we drive 65 miles per hour on the highway (velocity, or distance per unit time). Scientists use the metric system, which measures distance in meters (m), and velocity in meters per second (m/s).
What about acceleration? You might use the word “accelerate” in everyday language to describe something that is speeding up (like a car accelerating after a red light), or “decelerate” to describe something that is slowing down (like a car hitting the brakes). While velocity measures how fast distance changes, acceleration measures how fast velocity changes. That makes the units for acceleration sound a little weird: meters per second per second, or “meters per second squared” (m/s2). If something has a constant velocity, like a car using cruise control on the highway, then its acceleration is zero. If the velocity changes at all, the acceleration is not zero. For example, if a car is stopped at a stoplight, it has a velocity of 0 m/s. If it accelerates and two seconds later it has a velocity of 10 m/s, then its acceleration was 5 m/s2. Roller coaster riders can experience accelerations up to around 60 m/s2, while fighter jet pilots might experience accelerations up to 90 m/s2 for brief periods of time! Don’t worry if this all sounds confusing – just think of acceleration as how fast something is speeding up or slowing down (if it’s slowing down, the number will be negative).
What does all this have to do with smartphones and video game controllers? Acceleration can be measured with a tiny electronic device called an accelerometer. Modern smartphones contain built-in accelerometers, and can run apps that display the accelerometer readings. So if you want to explore motion in the world around you, all you need is a smartphone!
- Smartphone or tablet with internet access and permission to download and install an app
- Adult (to help verify and download the app)
- Ask an adult to help you search for an “accelerometer” app on a smartphone or tablet. There are plenty of free options available, but some apps may have ads or in-app purchases enabled.
- Get to know your accelerometer app. Some apps will just display a number on the screen, while others will display a meter or a graph. An app that lets you record data and automatically finds minimum/maximum values will work best for this project.
- Most accelerometer apps will display three acceleration readings, with labels like X, Y, and Z. These correspond to the motion of your phone in three-dimensional space, relative to your phone’s body.
- Usually, if you put your phone flat on a table with the screen facing up, then X acceleration is left to right, Y acceleration is forward and backward, and Z acceleration is up and down.
- Some apps may give you the option to display the combined or total acceleration, which combines readings from all three directions.
- Make sure the app is working: wave your phone around, and you should see the numbers change.
- Measure the acceleration of “typical” motions you make as you go about your day. Hold the phone in your hand or put it in your pocket as you walk around, sit down/stand up, go up and down stairs, etc. How big are the accelerations you measure?
- Now measure the accelerations of “fast” motions. Try jumping, running, twirling, or waving your arms around (be careful not to drop the phone!). How big are the accelerations now? How do they compare to some of the examples listed in the background section?
- Try dropping your phone a short distance onto a soft surface, like a pillow or a couch. What is the acceleration when the phone hits the ground? Do you think the acceleration would be bigger if the phone fell from a higher place or landed on a hard surface? (Don’t test that one!)
- How hard is it to move with a constant velocity? Put the phone down on a table and try pushing it in the X or Y direction. Can you get the corresponding acceleration reading to stay at zero?
Extra: try tilting your phone without moving it around. Do the acceleration readings change depending on which way you tilt the phone?
Extra: take acceleration readings on a playground. What happens when you go down a slide, swing on a swing, or climb on the monkey bars? Where do you experience the greatest acceleration?
Extra: take acceleration readings on different parts of your body. You can use a Velcro arm band to attach the phone to your arm or leg. Which part of your body experiences the biggest acceleration when you run? The smallest?
Extra: take measurements in a vehicle. What are the accelerations when you ride your bike or ride in a car?
Observations and Results
You should easily measure accelerations up to around 10 m/s2 as you go through regular motions while holding your phone. Smartphones, fitness trackers, and pedometers can track changes in these accelerations to count how many steps a person takes throughout the day.
The accelerations might be a little higher if you put the phone in your pocket instead of holding it, since it bounces around more. Faster motions, like swinging the phone, could result in accelerations over 50 m/s2 – the same accelerations felt by someone riding a roller coaster! Some electronic devices like laptops have a “drop detector” and will automatically power down to help prevent damage if they detect accelerations that are too high.
You might be confused if you discovered that you could get the acceleration reading to change just by tilting your phone. After all, the phone isn’t moving around – its velocity is zero, so shouldn’t the acceleration also be zero? This occurs because accelerometers also detect acceleration due to gravity*, which is 9.8 m/s2. As you tilt your phone, the direction of gravity relative to the phone’s body changes. Even if the phone is holding still, this will make the X, Y, and Z acceleration readings different depending on which one is pointing downward. This is how motion controls in video games (for example, tilting the controller to steer in a racing game) work!
*This is a bit of a simplification - see the Accelerometer Technical Note in the More to Explore section for a more detailed explanation.
Ask an Expert
- Speedy Science: How Does Acceleration Affect Distance?, Scientific American
- Accelerometer Technical Note, Science Buddies
- Science Activities for All Ages!, from Science Buddies