HotWheels Radar Gun

UPDATE (7/20/08) - Please excuse me modifying my post some two years later, but This post sees a lot of traffic. I did more experimentation after I wrote this, for the full review follow this link.

I just bought a Hot Wheels Radar Gun to see if it was at all useful in physics or physical science. The radar gun is hand-held. While it was designed for smaller hands than mine (go figure, 35 year old physics teachers aren't the target audience) it is still easy enough to hold and use. You have two unit settings kph and mph and you have the choice of getting straight speed or hotwheels car scale speed (1:64). I've of course been working in scale speed so I can be more precise (hopefully). When you pull the trigger you get a live readout of speed. When you let go it reports the maximum speed measured.

Experiment #1: Determining g using a falling object (a book)

• I held the radar gun above the book pointed down
• The book is held 2 m off the ground and dropped
• Velocity is taken in kph with the scale speed setting (1:64)
• Result - using the equation a=(v^2)/(2d) I get a value of 5.16 m/s/s. Only about a 47% error. (were shooting for 9.8 m/s/s)
  

Observations from Expt. 1

• If the book drifts out of the radar beam I don't get really big numbers. I've done this same experiment using Vernier motion sensors and if your target moves out of the "beam" the floor is shot given the impression that the object "teleported" instantaneously to the floor resulting in a really big velocity.
• The results are the same if the object is held really close to the gun as they are if you start further away. The Vernier motion sensor can't get a reading within 0.5 m or so.
• It was really hard to get consistent results. Tow out of three results would be the same, but the third would be wildly different.
  

Conclusions from Expt. 1

• The "reaction time" of the gun may be slow. This would explain the lack of teleportation velocity and may explain why I never got a true value for the maximum velocity of my falling object. It may very well be that the gun doesn't work well with accelerating objects and will give me more consistent results with constant velocity objects.
• Next step - Try known constant velocity experiment.

Experiment #2: Constant velocity. While riding my son's scooter I have my GPS unit. The settings are mph (same as the GPS) and scale speed for greater precision. When in real speed, speeds are reported only to the nearest mile per hour.

• Read GPS unit wile attempting to not fall off and not run my son down (while going between 8 and 9 mph).
• My son shoots me with the radar gun staying in my path just long enough to get a good reading before diving to safety.
• Compare the results.

Observations from Expt. 2

• The results are much more consistent than in Experiment #1.
• The numbers from the GPS unit and radar gun are fairly similar. It's tough to get a good reading from the GPS unit while not running my son down.
  

Conclusions from Expt. 1 and 2

• My inital conclusions seem to be confirmed. A constant velocity seems to give a better answer. There was a 13% error between the radar gun and the GPS velocities. While this seems like a lot, I am not really concerned. I know neither the error rate of the radar gun nor the error in the velocity estimate of the GPS unit. The error inherent in both devices may be compounded. It is impossible to know without further testing.
• The radar gun may in fact be not too bad. After the first experiment I wasn't so sure.

The Next Step - Run an experiment with both the radar gun and the Vernier motion sensor at the same time. I know the Vernier system works. This should give me enough data to confirm whether or not the Hot Wheels Radar Gun is worth anything. My feelings right now are it is worth the $30-35 spent, but only for measuring constant velocity. If you're planning on non-constant velocity this is probably not the tool for you. In the next week or two my classes will be doing a lab determining the terminal velocity of falling coffee filters. I'll make one group use the radar gun along with the vernier sensor. Unless I get ambitious between now and then and do it myself. Either way, I'll report the results here when done.

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