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Re: test problem



I continue to need help with this. I didn't think is was slow until I came upon this problem.

I changed the original problem slightly to account for some of the difficulties mentioned previously. The problem now reads:

"This question is related to the conservation of energy lab: a 3.0 kg mass is hung from a spring which is then observed to stretch 15 cm. The mass is then pulled down to 25 cm from the unstretched length of the spring and released from rest.
a) find the spring constant of the spring
b) find the total potential energy (spring and gravitational) of the system after it is stretched down. (Define the origin to be the bottom of the unstretched spring.)
c) Use conservation of energy to find the speed of the mass as it passes through the 15 cm point"

In this way we avoid the "sproing" effect which has been mentioned. It does not avoid the ambiguity of gravitational potential energy.

I am beginning to wonder if I have no understanding of mechanical energy because when I work the problem this way I obtain a negative kinetic energy. This is the same difficulty the students had in my first email.

Am I trying to make a difficult problem too simple by using Ug=mgh and Us=1/2 kx^2 rather than integrating work done by the spring and by gravity? Can someone suggest a change or perhaps a better problem altogether? Originally I was attempting to include a problem which would give an advantage to people who had done the lab like they were supposed to. Now I am wondering if even the whole lab was bogus.

Thanks as always

Justin
(who is "only" a high school teacher but thought he understood mechanics)