# Fresh Ideas for Your Relative Speed Discussions

Whether we realize it or not, all velocity/speed measurements are made relative to some other object. When your car's speedometer reads 55 mph, the assumption is that the change in position of the car compared to the stationary ground is 55 miles every hour. However, not all velocity/speed measurements are made relative to a stationary object. The Constant Velocity cars allow students to experimentally investigate relative speeds as the cars with different speeds move in the same and in opposite directions. In this demonstration, Dr. Joel Bryan of Ball State University explores Relative Velocity (Speed) with cars: Moving Opposite Directions (Toward Each Other); Moving Opposite Directions (Away From Each Other); Moving Same Direction (Faster Car in Back); and Moving Same Direction (Faster Car in Front).

Download attached lab with student worksheets!
Relative Velocity Lab PDF (323Kb)

There are numerous constant and relative velocity and acceleration experiments that can be demonstrated when you have a device like the constant velocity car, which runs at a constant speed. If you also have a device like the pull-back car, which accelerates at a constant rate, you can cover a wide range of discussion topics in kinematics. Here are just some of the many resources available on the Arbor Scientific website to help you make this important set of topics fun and engaging for your students:

Constant Velocity Car Video – Discusses measuring constant velocity, the different tools you can use to measure, how to change speeds with the constant velocity car, uniform circular motion and the introduction of centripetal acceleration, and constant acceleration:

Constant Velocity Car Datasheet (Note: This link takes you to the Constant Velocity Car Product page, where you can click on the "Datasheet" link) – This datasheet provides all of the specifications for the constant velocity car, as well as numerous experiments you can use with your students.

Student Lab – Measuring Constant Velocity – This lab introduces the concept of graphing the position vs. the time of a moving car to determine the car's velocity.

Student Lab – Spark Timer Motion – This lab introduces the concept of using a spark timer to record the motion of two toy cars.

Student Lab – Car & Ramp: Speed and Acceleration – While this particular lab does not use the constant velocity or pull-back cars, it does show how to use photogates to find the speed and acceleration of an object in motion. These devices are useful when attempting to take more accurate measures of time that are too short to measure with a stop watch.

Collin Wassilak

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