A science video blog,1 which has gone viral, shows a wooden block shot by a vertically aimed rifle. The video2 shows that the block hit dead center goes exactly as high as the one shot off-center. (Fig. 1). The puzzle is that the block shot off-center carries rotational kinetic energy in addition to the gravitational potential energy. This leads a majority of the bloggers to claim that the block shot off-center should not go as high as the one shot dead center. Others have claimed that the energy tied up as rotational energy is insignificant and the two blocks should rise to the same height within experimental error.

Topics
Mechanical energy, Friction, Gravitation, Newtonian mechanics, Rotational dynamics, X-rays, Atomic and molecular collisions, Educational assessment
1.
The science video blog is Veritasium and the YouTube link is https://www.youtube.com/watch?v=vWVZ6APXM4w.
2.
A follow-up Veritasium video where the misguided conservation law is invoked can be found at https://www.youtube.com/watch?v=BLYoyLcdGPc.
3.
An interactive video vignette was presented by David Jackson and Priscilla Laws at the AAPT winter 2014 meeting, http://www.compadre.org/psrc/items/detail.cfm?ID=13324.
4.
Hunter G.
Close
 and
Paula R. L.
Heron
, “
Student understanding of the angular momentum of classical particles
,”
Am. J. Phys.
 
79
,
1068
(
Oct. 2011
).
https://doi.org/10.1119/1.3579141
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Crossref
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5.
The ballistic pendulum problem, where a bullet gets lodged in a freely suspended pendulum, which subsequently rises up, has caused grief to countless generations of unsuspecting (read “unprepared”) students. Once they start the problem with the misguided conservation law that the total kinetic energy of the bullet must reappear as the kinetic energy of the bullet-block combo, their fate is sealed! This classic gem of physics problems has been around for generations before the bullet-block video made its appearance. A mutant variant of the ballistic pendulum problem rears its ugly head as an initially charged capacitor that is then connected to a similar but uncharged capacitor. A misguided assumption is that the total electrical energy of the first capacitor will simply be redistributed between the two capacitors. The correct conservation law is the conservation of charge.
© 2015 American Association of Physics Teachers.
2015
American Association of Physics Teachers
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