Average Velocity

• Distance divided by time results in average velocity.Thinkstock/Comstock/Getty Images
  
  Velocity is defined as a change in position in a given time. If two towns are separated by 60 miles and a train makes it from one town to the next in an hour, then the velocity is 60 miles per hour. The simplest method of measuring velocity is with a "yardstick" and a clock. For a runner on a track, for example, a simple measurement of velocity can be made by starting a stopwatch as the runner begins and stopping it as the runner passes the 100-yard mark. The average velocity is 100 yards divided by the time difference. The same principle can be used to measure the velocity of a walker, a bicyclist, a driver, a train engineer.
  
  

Changing Velocity

• Although the previous technique is completely accurate, it doesn't reveal details of motion. To get the details of changing velocity, the measurement needs to be made on a smaller scale. For example, if you tape paper along the edge of a six-foot board and set it up at a slight angle, it can serve to record position changes with respect to time. Then release a ball at the top of the board while holding a stopwatch. Each second, make a mark on the paper at the spot the ball is passing. Then measure the distance between each mark. The velocity at each second is the distance between marks divided by one second. You can graph the velocity at each second to determine the changes.
  
  

More Resolution

• You can use the same board and ball to record velocity even more accurately. If you set up a ruler along the edge of the board, you can use a video camera to record the ball as it moves past. At each frame of the video, make a note of the position of the ball -- where it is on the ruler. Each frame of your video is exposed at a regular rate -- often 30 Hz. The velocity at every thirtieth of a second is equal to the distance traveled between frames divided by the time between frames, one-thirtieth of a second. Now you can graph the changes in velocity on a finer timescale.
  
  

Creative Methods

• Measuring the change in a signal's frequency can accurately determine speed.Thinkstock/Comstock/Getty Images
  
  If you've ever heard a train whistle lowering its pitch as the train approaches you, you've observed the principle behind the radar gun that measures the speed of a baseball pitch -- or of your car on the highway. The frequency of a wave shifts if the source of the wave is moving along the direction in which it's being detected. Radar and laser detectors put out a frequency and bounce it off an object. The detector measures the reflected beam. If the object is moving, the frequency will be shifted. The frequency shift is directly related to the velocity of the object.
  
  

Velocity and Speed

• One nuance to be aware of: Velocity and speed are not exactly the same thing. Speed measures how much distance an object travels in a given amount of time. Velocity measures how much distance an object travels in a given direction in a given amount of time. For many measurements, you choose the direction of measurement to match the movement of the object, so speed and velocity are the same thing. You can use the exact same methods to measure speed in more than one direction.