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Date: Fri, 23 Apr 2010 17:52:00 -0700
From: Bernard Cleyet<bernardcleyet@redshift.com>
Subject: [Phys-l] momentum dissipation?
To: Forum for Physics Educators<phys-l@carnot.physics.buffalo.edu>
Message-ID:<20094BA7-6872-433E-B77E-105C72548FAD@redshift.com>
Content-Type: text/plain; charset=windows-1252
HS teacher's lab exercise:
I have a HotWheels track with a loop. For this I do an energy/circular motion demo. We calculate the minimum height to release the car so that it will make the loop. This usually comes within 10% of the predicted value. The HotWheels car does put a tiny amount of energy into rotating the wheels, we neglect that. It also will have a little internal friction and will rub against the side of the track. This accounts for the error. It does not affect the momentum results in the dart lab** because momentum does not dissipate as heat. (bc emphasis)
** ?Tailgated by a Dart?. Attach velcro to the back of a HotWheel and to a suction tip dart. The students shoot the dart at the car and it hits and sticks (usually!). They measure the time it takes the car/dart to coast to a stop. Divide the distance it coasts by the time and you have the average velocity. This times 2 is the initial velocity. Now use momentum conservation to solve for the initial velocity of a dart. Shoot the dart through 2 photogates to check. Make sure you have a large variety of cars to choose from. Cars with a wide back end work the best.
bc requests comments