A question of real gravity Part2

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Last week we looked at the question of dropping a bullet and firing one horizontally from the same height, and while we conceded that there would be a slight difference in the time they come to earth, we believe that the difference would be undetectable. A second quibble on the subject of shooting bullets came from the same episode, and is also related to guns...

The number of quibbles that the elves receive increases greatly when we contradict a popular source of information. It goes without saying that wikipedia is one, but another is the excellent popular-science program "Mythbusters." Mythbusters has shown in one of its episodes that bullets shot into the air would not endanger those who fired them, but we think otherwise: while Mythbusters is a great show for getting people interested in science; its experiments are not scientifically rigorous.

The elves' figures come from the work of Julian Hatcher, noted firearms expert and pioneer in the forensic identification of firearms and their ammunition. Not being able to easily do practical experiments with real firearms, it's easier to refer to the work of someone who has and who knows his subject well.

Having read the MythBusters transcript we can see a pretty major flaw in their experiment. Their two main experiments don't actually involve firing bullets into the air at all. Rather the bullets are dropped (either into a wind tunnel or from a platform).

Bullets fired from a gun will be rifled which causes them to spin about their long axis in order to give them stability. A bullet which reaches the top of its trajectory still spinning will remain in the upright position due to angular momentum and, therefore, start to fall back down bottom first (as mentioned in the work carried out by Major Hatcher). A bullet which is simply dropped will not have this kind of stability and so will be much more likely to tumble, thereby reducing its terminal velocity and making it less deadly.

It's also interesting to note that of the two firearms experts giving an opinion in this debate (the aforementioned Julian Hatcher and Dr. David G Mohler who was consulted in the Mythbusters show), both were of the opinion that falling bullets are deadly.

Currently the only people who are saying falling bullets aren't deadly are the Mythbusters presenters and those who have watched the show.





If you see anything in QI's input that doesn't seem quite right, get in touch by e-mail at elves@qi.com or visit our talkboards at qi.com/talk

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Todays QI Fact Of The Day states, "A bullet fired straight up in the air loses about 90% of its speed on the way back down, giving it the energy of a brick dropped from a height of about four feet." Call me pedantic, but surely the bullet has lost 100% of its speed before it starts down - under the influence of gravity and is therefore accelerating at 1g until it reaches a terminal velocity determined by its air resistance. At this point will not the energy be determined by its mass?

Yes I agree completely with Testudinarian on the bullet fired upwards scenario. Also there was very muddy logic from Stephen and the panel on gravity and the subject of dropping differing weights from the same height. This is generally true but things will only fall at the same speed as each other in a perfect vacuum. A ton of lead will clearly hit the ground much earlier than a ton of feathers on the Earth. Occasionally on QI yer actual physics are treated in a bit of an amateurish way. Good program though!

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This page contains a single entry by eggshaped published on December 5, 2010 10:34 PM.

A question of real gravity Part1 was the previous entry in this blog.

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