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Re: [Phys-l] momentum dissipation?



If the car is coasting to a stop, there is friction. If there is friction, then momentum is being transferred into the track, then the table, the floor, and into the Earth.

The transfer of momentum into the Earth is a confounding factor that makes momentum problems difficult for students. On the one hand, we drill into them that momentum is conserved. On the other hand, we talk about cars coasting to a stop - a clear change in momentum of a single body.

Unless we are very clear that A) the coasting car is not a closed system; and B) when it comes to momentum, the Earth giveth and the Earth taketh away, students can never form a correct cognitive model of Newton's 3rd Law and its consequence, C of M. The students then can only plug numbers into formulas for known situations, and have no deep conceptual understanding of what is really happening.

They need to be exposed to a lot of situations where it appears that momentum is not conserved (like a car braking, or for that matter speeding up), and given the understanding that it must, find out where it went to or came from.

Scott

On 4/24/2010 12:00 PM, phys-l-request@carnot.physics.buffalo.edu wrote:
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