Re: [Phys-l] The History of MASS

[Bernard Cleyet <bernardcleyet@redshift.com>]

Unfortunately, some wise guy HS student may point out that what they are 
experiencing is the static friction, which also increases w/ the normal 
force, and is very likely, whatever, the normal force above a rather low 
minimum, to be greater than the kinetic, or in the case of wheels, 
rolling friction.   I still think the only honest method is w/ a spring 
scale *.  It will also show the difference between the static and 
rolling friction and the various masses.  The following site is rather 
good regarding friction especially the graphs and the automobile discussion.

http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html


* Or  Jeff Schnick's experiment (no friction to muddy the experiments).

bc, who arrogantly pontificates having never taught below 12th grade.

p.s. I just now remember I have a 0 => 30# scale.  It, my rusty old 
little red wagon, concrete construction blocks, and video camera will 
get used tomorrow.

Steve Clark wrote:

> What's really of interest here is the force required to start the  
> wagon moving. While the friction force is certainly larger with the  
> added mass, the point would be made if the students' attention was  
> directed to the starting (accelerating) force and not the force  
> required to maintain equilibrium. I'd postpone any discussion of  
> changing frictional forces to a later date.
>
> I have my high school students teach elementary kids in the spring  
> for a year end project. They seem quite good at getting this point  
> across to their young students.
>
> Steve Clark
>
> On Oct 20, 2006, at 7:57 PM, Bernard Cleyet wrote:
>
>  
>
> > 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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