Questions

1. What drove the decision for dropping the bomb on Hiroshima?

2. Who were the observer scientists on the plane?

3. Considering that the Hiroshima attack set a historic precedence, do you think that Hiroshima should/could have been avoided?

Nagasaki

Why drop a second bomb?Why drop the second bomb at all? Hiroshima has been justified as a way tosave the lives that an invasion of Japan would cause. It has been explained asa way to impress the Russians and ensure American superiority in Asia.

By August 1945 the Japanese were all but defeated. The Soviet declaration ofwar was scheduled for August 15. Truman wrote in his diary about this event,"When this happens, Fini Japs."On August 9, 1945 the Japanese were reeling from the effects of the bomb onHiroshima. Their surrender was inevitable before the 15th.Why drop a second bomb?Is it revenge for Pearl Harbor or is the only reason that:

the Americans had two bombs?“When you have to deal with a beast you have to treat him as a beast."–President Harry S. Truman, August 1945

"Now is the time to exterminate the Yellow Peril for all time… Let the rats squeal."—Congressman Charles A. Plumley, August 1945

Why Nagasaki?Nagasaki was at the bottom of the pre-identified list of the Target Committee,weather conditions dictated the choice. On the second target list, it wassecond choice after Kokura.

The reason for being on the target list at all was a concern for psychologicaleffect. Nagasaki was added shortly before the first mission as the last on thelist of alternate targets. Kyoto was considered to be the first choice becauseof its history, as the ancient capital, and "the advantage of the people beingmore highly intelligent and hence better able to appreciate the significance ofthe weapon.“

These factors were present in Nagasaki as well. The city had been an ancientcenter of trade with foreign countries, first with China, Vietnam and southAsia. Later when trade with the outside world was cut off, Nagasaki remaineda Portuguese and Dutch outpost. Nagasaki was a religious center forCatholicism as well as Buddhism. As a result literacy was high. In addition itheld vital war related industry, being the home of the Mitsubishi Aircraft plantand the Ohashi Arms factory.

The old Nagasaki

Old trading center founded by the Portuguesein the 16th century on the site of a Japanesevillage in the south of the Japanese Isles. Verymountainous region with narrow harbor inlet,later site of a Dutch enclave Dejima during theperiod when the country was closed to foreign,access in 1641–1853. In 19th century site ofthe early Japanese naval training academy.

Topography of NagasakiThe topography ofNagasaki made it anideal site for testing theshock front behavior andshock reflection addinga new parameter to theimpact analysis of anuclear blast. The targetpoint was chosen to beright in the center of avalley, site of thesuburban town Urakamislightly above the actualcity center. Mountainsreflect a shock, valleyschannel the shock incertain directions!

Shock Front AnalysisDecline of shock intensity with

time and distance

Shock Enhancement & Channeling

Multiple shock bouncing is possible, causing considerable enhancement of the initial destructive power of explosion

Nagasaki

On August 9, 1945 at 9:44 a.m. “Bockscar”, a B-29 planecarrying Fat Man, the world's third atomic bomb, arrivesat its primary target, Kokura. The city is covered in hazeand smoke from an American bombing raid on a nearbycity. Because of lack of target visibility “Bockscar” turns toits secondary target Nagasaki. At 11:02 a.m. the world'sthird atomic bomb explosion devastates Nagasaki, theintense heat and blast indiscriminately slaughters itsinhabitants, 74,000 people died immediately. 10,000Catholics died since target was suburb Urakami, a centerof Japanese Catholicism.

Surveying the results

Report of the Pilot

The effects of the blast

Urakami Cathedral was the largest church in the entire Orient. TheUrakami parish counted 14,000 members.

Cultural and religious centers

Destruction

Mitsubishi Armament Factories

Distance from Ground Zero (km) Killed Injured Population

0 - 1.0 88% 6% 30,900 1.0 - 2.5 34% 29% 144,800 2.5 - 5.0 11% 10% 115,200

Total 22% 12% 173,800

Shock emergenceFriedländer formula for the timedependence of pressure in atransversing shock at a fixedlocation with respect to shockorigin

tteptp t

t

r 1)(t*

5

2

rt

WTaylor equation, derived phenomenological fromobservation of Trinity fireball expansion. W isthe released energy at distance r and time t withair density and specific heat ratio of air

Expectations

mmkg

ssmkg

mkgskTontWr

rt

W

840/2.1

110184.4224.1

/2.11224.1

5/1

3

22

2125/1

3

25/12

5

2

Fat Man had an explosive power of W=22kT

With a typical air density of 1.2 kg/m3 and a typical 1.4 the shock front expands in 1 s by r 840 mNagasaki spreads over area of ~10km2 4sqm its destruction was a matter of a couple of seconds.

Definition: 1 ton of TNT = 4.184 x 109 joule (J).

Pressure Units

v

de

Pa bar at atm Torr psi

1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 145.04×10−6

1 bar 105 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.5037744

1 at 0.980665 ×105 0.980665 ≡ 1 kp/cm2 0.96784 735.56 14.223

1 atm 1.01325 ×105 1.01325 1.0332 ≡ p0 760 14.696

1 Torr 133.322 1.3332×10−3 1.3595×10−3 1.3158×10−3 = 1 mmHg 19.337×10−3

1 psi 6.895×103 68.948×10−3 70.307×10−3 68.046×10−3 51.715 ≡ 1 lbF/in2

Pressure units

pascal bartechnical 

atmospherestandard 

atmospheretorr pound per square inch

pressure is defined as: force/area and is the main source of mechanical destruction of buildings and other human structures

Shock Velocities2/1

00 2

11

ppcU

U: shock velocityc0 : speed of sound c0 343 m/s : ratio of specific heat in transporting medium, air 1.4p : peak overpressurep0 :ambient pressure ahead of shock (air pressure: p0 1atm)

00 7

61ppcU

for a peak over pressure of p 20 atm, the shockfront velocity is U 1461 m/s, that is a supersonicshock, reaching you before you can hear the blast!

pressure is defined as: force/area

Destruction Analysis

with distance from ground zero

Between triumph and revenge feelingsThe US Press

Japan’s Unconditional Surrender

September 2, 1945On board of battleship Missouri in Tokyo Bay

Gen. MacArthur referred to the nuclear bomb attacks on Hiroshima and Nagasaki, saying they had "revised the traditional concept of war".

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