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[Phys-l] Mass vs. Weight demos (was The History of Mass)




I just went outside with a bottle cap. I raised both hands and dropped
the bottle cap from my left hand when it was at the same elevation as my
right. I was easily able to get my right hand under the bottle cap and
catch it at waist level. This suggests that I can cause my hand to
accelerate at a greater rate than objects in freefall accelerate. So I
figured that if I threw the bottle cap down toward the ground, I would
be causing it to accelerate at a greater rate than it accelerates in
freefall. I threw it to the ground to see what it would feel like to
cause an object to accelerate at a rate greater than it accelerates in
freefall. Then I found a half a brick under my deck. I tried the same
thing with it. Interestingly enough, despite the fact that the earth
was pulling downward on the half-brick harder than it was pulling
downward on the bottle cap, I could feel that I was pushing much harder
on the half-brick to make it accelerate downward at a greater rate than
it would in freefall. Furthermore, despite the fact that I was getting
more help from the earth's gravitational field, it didn't seem to me
that I did as well with the brick as I did with the bottle cap. By that
I mean that I think the bottle cap left my hand with a greater speed
than the brick did.

Jeff Schnick

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard
Cleyet
Sent: Friday, October 20, 2006 7:58 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] The History of MASS

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.