Re: [Phys-l] The History of MASS

[Bernard Cleyet <bernardcleyet@redshift.com>]

JMC got it, but I'll add.

It's likely that the wagons an elem. or middle school likely has  will 
have sig. friction.  So 200 # on a 20# wagon will significantly increase 
the friction.  So, what ever, PS's experiment will always measure the 
friction force as a function of the weight.  I suggest she obtain a high 
K spring scale attach it to the wagon and have a student pull such that 
it has a constant reading.  If the school is affluent, then a Vernier 
force detector and their ultrasonic position detector together may be 
used.   Thereby, w/ a little algebra defining a new column, rather 
constant force is unnecessary.

bc, who'd try it out if he didn't have to prepare for a talk early next 
month.

John M Clement wrote:

> Unfortunately this explanation has some problems that can lead to 
> misconceptions.  The first one is that the frictional force increases. 
> But if the frictional force were small then just pulling the wagon at 
> a constant velocity would be an extremely low force for either 
> situation.
>
> A better explanation would be to have two air supported objects with 
> different masses.  Air hockey pucks would be OK, or students on air 
> carts and explore how it takes more force to start or stop them. 
> Arnold Arons advocated large blocks of dry ice on a level surface. 
> This is actually better than air hockey pucks because there is no 
> obvious source of power to confuse students.  Or very low friction 
> carts on level tracks with and without extra mass.
>
> John M. Clement
> Houston, TX
>
>  
>
> > Your example of pushing the car leads to my explanation to younger 
> > students (grade school) differentiating mass and weight.  They all 
> > remember pulling a wagon, so I ask them if it is easy to pull an 
> > empty 
> > wagon on a level sidewalk and they all agree that it would be.  Then 
> > I 
> > ask them to imagine 5 of their buddies hopping into the wagon and 
> > they 
> > giggle at the image.  I ask them to imagine pulling the wagon with 
> > that 
> > many children in it and whether it would be harder to pull than when 
> > it 
> > was empty.  They can all agree it would be more difficult.  I tell 
> > them 
> > that it is more difficult to move because of the additional MASS of 
> > the 
> > added bodies, not the weight as they are not trying to lift the 
> > wagon. 
> > Of course actually doing the activity would be better, but I usually 
> > have just an hour with them to give them a taste of "all" of 
> > physics.
> > Pati
> >
> >    
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