Key Concepts
Physics
Motion
Position
Velocity
Acceleration
Measurement

Introduction
Have you ever played a video game with a controller that used motion controls? Do you ever wonder how sometimes a smartphone seems to "know" if you're moving? How do these electronic devices measure motion? Try this activity to find out!

Background
You're probably familiar with the units we use to measure distance and velocity. In the U.S., we might say someone is 5 feet 11 inches tall (measuring distance) or that we drive 55 miles per hour on the highway (measuring velocity, which is a unit of distance during an amount of time). Scientists use the metric system, which measures distance in meters (m) and velocity in meters per second (m/s).

To reach a certain velocity, an object will often need to speed up or slow down. You might use the word "accelerate" to describe something that is speeding up (such as a car accelerating after a red light) or "decelerate" to describe something that is slowing down (such as a car applying the brakes). We can explore these ideas even further. Whereas 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, such as a car using cruise control on the highway, 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, its acceleration was 5 m/s2. Roller coaster riders can experience accelerations up to approximately 60 m/s2, and fighter jet pilots might experience accelerations up to 90 m/s2 for brief periods of time! Don't worry if this all sounds confusing—you can 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 small electronic device called an accelerometer. Most smartphones these days 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!

Materials

• Smartphone or tablet with Internet access and permission to download and install an app
• Soft surface, such as a pillow, bed or couch
• Velcro band (or other method) for attaching the phone to your arm or leg (optional)
• Vehicle to ride on or in, such as a bicycle, car, bus, and so on (optional)

Preparation

• Get to know your accelerometer app. Some apps will just display a number on the screen, whereas others will display a meter or a graph. An app that lets you record data and automatically finds minimum and maximum values will work best for this project.
• Most accelerometer apps will display three acceleration readings, with labels such as "X," "Y" and "Z." These correspond to the motion of your phone in three-dimensional space.
• Usually if you put your phone flat on a table with the screen facing up, "X" acceleration would be any movement left to right, "Y" acceleration would be any movement in the direction of the top or bottom of the device, and "Z" acceleration would be any movement up and down (above or below the table surface).
• Some apps might give you the option to combine readings from all three directions and will display the combined or total acceleration.
• Make sure the app is working: wave your phone around, and you should see the numbers change.

Procedure

• 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, and so on. 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 large are these accelerations?
• Try dropping the phone a short distance onto a soft surface, such as a pillow, bed or 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 difficult 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 the 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 from different parts of your body. You can use a Velcro (or other) 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 (don't look at the phone while you're riding!) or ride in a car or on a bus?

Observations and Results
You should easily measure accelerations up to approximately 10 m/s2 as you go through regular motions while holding a 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 because it bounces around more. Faster motions, such as swinging the phone, could result in accelerations of more than 50 m/s2—close to the same accelerations felt by someone riding a roller coaster. Some electronic devices, such as laptops, actually have a "drop detector" and will automatically power down to help prevent damage if they detect accelerations that are too high. (Again, don't test this one!)

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.

More to Explore
Accelerometer Technical Note, from Science Buddies
Speedy Science: How Does Acceleration Affect Distance? from Scientific American
Showing Science: Watch Objects in Free Fall, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies