Projectile Motion
Forces and Motion

Murphy’s Law

Stories from Physics for 11-14 14-16 IOP RESOURCES

An analysis has been carried out on the dynamics of pieces of toast in order to test the prediction of Murphy’s Law that toast is more likely to land butter side down than up. In experimental work to test the hypothesis, the coefficient of static friction for bread (0.29) was found to be slightly higher than for toast (0.25). Matthews reports that toast is indeed more likely to land butter side down but argues that one factor that accounts for this result is human height, which determines the height of tables. A table 3 m high is recommended to prevent butter-side-down collisions. However, the maximum safe height of a humanoid creature that limits the danger of fatal falls is calculated to also be around 3 m and hence Matthews concludes despairingly: ‘all human-like organisms are destined to experience the ‘tumbling toast’ manifestation of Murphy’s Law because of the values of the fundamental constants in our universe’. The author dismisses two solutions to the toast problem: the building of tables which are 3 m high is ‘impractical’ and he argues that the reduction of toast to 2.5 cm squares is ‘unsatisfactory’. Matthews suggests that toast that is beginning to fall should be given ‘a smart swipe forward with the hand’ to limit the period over which the toast is exposed to a gravitational torque.

There is another intriguing connection between acceleration and Murphy’s Law. There is a hypothesis that the phrase originates from a development engineer, Edward Murphy, who worked on the rocket sled tests in which John Stapp participated, described on page 12. During one test, an assistant wired the strain gauges that would measure deceleration back-to-front, resulting in a loss of data. Murphy’s expression of exasperation with his assistant might be the original use of the phrase.


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