Archive for October, 2013

A 3.0 m long steel chain is stretched out along the top level of a horizontal scaffold at a construction site,?

A 3.0 m long steel chain is stretched out along the top level of a horizontal scaffold at a construction site, in such a way that x = 1.6 m of the chain remains on the top level and y = 1.4 m hangs vertically. (See the figure.) At this point, the force on the hanging segment is sufficient to pull the entire chain over the edge. Once the chain is moving, the kinetic friction is so small that it can be neglected. How much work is performed on the chain by the force of gravity as the chain falls from the point where 1.6 m remains on the scaffold to the point where the entire chain has left the scaffold? (Assume that the chain has a linear weight density of 32 N/m.)

I’d look at this situation this way:
The picture of chain lying on scaffold at the start has the center of mass (CM) of hanging part of chain at y/2 = 1.4/2 = 0.7 m from top of scaffold. At the finish, the entire chain is hanging in air with its CM at a distance = (1.6 + 1.4)/2 = 3/2 = 1.5 m from top of scaffold. So the change in CM position during the entire downward movement equals: 1.5 – 0.7 = 0.8 m.

In first picture described above,
the gravitational force on the hanging chain = (32)(1.4) = 44.8 N.
In "last picture" entire chain hangs vertically off scaffold,
gravitational force = chain weight = (32)(3) = 96 N.
The average weight of chain pulling downward
during the chain’s movement = (44.8 + 96)/2 = 70.4 N

Work done by the force of gravity on chain = (70.4)(0.8) = 56 J ANS

1 comment - What do you think?  Posted by admin - October 21, 2013 at 4:52 pm

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Is it educationally unjust to separate students by ability and intelligence in high school?

In nearly all American high schools there is a tracking system that places certain students in various classes by category: special ed, general, and honors/AP.

This whole idea of "tracking" traces its origins in the 1950s when school administrators realized that the traditional college preparatory did not suit average students. Students identified with college potential were tracked into English Literature and history classes, while those believed to have less were given a vocational track. This was when classes such as "woodshop" and "home economics" emerged. Woodshop was mainly for male students who would later become part of the manual workforce and home economics was for female students believed to one day become stay-at-home mothers. Placement into the tracking system was based on an IQ verbal test. In Los Angeles, the majority of the Latinos and African Americans were tracked in the vocational path.

Tracking has been taken to the extreme degree in the recent past with the spread of magnet schools. Magnet schools, the cream of the crop in public education, boast high test scores and excellent academics. While magnet schools are public, they often only benefit middle to upper class non-disadvantaged groups. One criticism of magnet schools is that they siphon off the very best students from average schools, leaving environments where students aren’t able to learn from their more capable peers.

Is there a question here?

It’s a good idea to provide specialized paths to people – otherwise you end up with a one size fits all policy where one size doesn’t fit all. Those who are interested in excelling in school can’t, and those who are struggling sink even further.

What’s not fair is if people are streamed on criteria that aren’t fair – e.g., race, sex, income level etc. But streaming on ability or giving people specialized curriculum to enhance their interest makes sense – you don’t develop expertise through mediocrity.

3 comments - What do you think?  Posted by admin - at 4:52 pm

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