Re: [Phys-l] momentum dissipation?

[curtis osterhoudt <flutzpah@yahoo.com>]

Absolutely. 

   Though we "experts" tend to take it for granted, the ability to scale the problems so that interactions become viscerally obvious doesn't seem common for students. I once gave a pre-lab quiz in which the braking of a bicycle seemed to magically dissipate momentum. Though many students got the *answer* right, only one took the most pleasing (to me, anyway) step of imagining a much smaller Earth (maybe tens of meters across), whereupon the momentum transfer becomes more visible.

 /**************************************
As a species, we are forever sticking our fingers into the electric socket of the Universe to see what'll happen next. It's a trait that'll either save us or kill us, but by god it's what makes us human beings. I'd rather be in the company of people who look at Mars than people who contemplate humanity's navel -- other worlds are better than fluff. ~~Sir Terence David John Pratchett
***************************************/




________________________________
From: Scott Orshan <sdorshan@aol.com>
To: phys-l@carnot.physics.buffalo.edu
Sent: Sun, April 25, 2010 7:57:47 AM
Subject: 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
>    
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