The Proof of Innocence

Photo by Tripp.

Recently, Dmitri Krioukov, a senior research scientist at University of California - San Diego, was issued a traffic citation for failing to stop at a stop sign. Krioukov claims that he did, in fact, stop, and that the officer made a mistake in assuming that he did not.

Rather than pay the $400 fine, Krioukov decided to put his skills to use and wrote a paper explaining his theory. The four-page paper, titled "The Proof of Innocence" is posted online. 

1. Krioukov claims that his car might have appeared to have not stopped at the stop sign, proved three conditions are true. What are these three conditions?
2. Krioukov frequently uses the term "angular speed." What does he mean by this, and how is it related to linear speed?
3. News articles about the issue have been posted by the Los Angeles Times and (a different version) by Fox News. Read about the situation and comment on whether or not, in your opinion, Krioukov  should have to pay the $400 fine.

Pulleys Everywhere?

Photo by emkladil.

We've been studying simple machines in class, and we've talked about lots of examples of each type. In this post, I'd like us to focus specifically on pulleys:

1. Sometimes, a pulley simply changes the direction of a force without increasing the force. Therefore, the mechanical advantage (MA) is 1. Other times, pulleys can be used to increase a force (and have mechanical advantage of much greater than 1). How can this be? How can you tell just by looking at the pulley whether or not it will increase the applied force?
2. Where are pulleys used in practical applications?
3. Look carefully at the pulley in the photo at the top of the page. How might this pulley be used?