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Online Videos - Fake or Not? Stuff from the InterWebs Rhett Allain Dot Physics (dotphys.net) Southeastern Louisiana University

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The Physics Suppose an object is in free fall Only one force (probably can ignore air) The acceleration is -9.8 m/s^2 down

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Measurable Stuff Horizontal velocity = constant Vertical position should be parabolic (with time)

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The Tools Tracker Video Analysis Get x, y, and time data from each frame http://www.cabrillo.edu/~dbrown/ tracker/

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Video Analysisability Score Stationary camera and background Camera does not zoom Motion perpendicular to the camera You can see the object in each frame No repeating frames Downloadable Interlacing

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Case 1: Elephant on a trampoline

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Analysis Scale = 1 trampoline

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Verdict: Fake Not a constant acceleration

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Case 2: Kobe number 1

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Problem - not a point object plot head, truck, arms, thighs, calf Kobe = 1.98 m

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Horizontal Motion Verdict: Could be real, but faked

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Case 3: Kobe number 2 Jumping over a pool of snakes

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vertical motion

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horizontal motion Verdict: No reason to say it is fake (except that it is crazy)

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Case 4: Movie Up! Boy throws out GPS Verdict: Hello? Fake.

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More Physics: Work-Energy Principle Human G-force (acceleration) tolerance Time (min) +Gx (eyeballs in) -Gx (eyeballs out) +Gz (blood feet) -Gz (blood head) 0.01 (sec) 3528188 0.03 (sec) 2822147 0.1 (sec) 2017115 0.3 (sec) 151294.5 1 (sec) 11973.3

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Case 5: Hancock Not nice to call people names Not nice to throw people like that

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The boy was in the air for 23 seconds. So, how fast did Hancock throw him? What if there is no air resistance? if acceleration = g, and no air resistance - then initial speed = final speed v initial = 113 m/s or 250 mph If the boy is accelerated over 2 meters, what would it be?

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Air Resistance At 250 mph, air resistance plays an important role C is a coefficient of air resistance Force is in the opposite direction as v Depends on other stuff also density of air cross section area shape

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Finding C Assume that the boy would fall at a terminal velocity similar to an adult. For a falling person: At terminal velocity, air resistance = gravity

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How to model this motion? Problem: Net force is not constant. Break it into pieces of time that have about constant force. - Numerical Calculation Plan: Calculate the force Calculate the acceleration Update velocity Update position Update time Plot stuff

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How fast to be in the air for 23 seconds? What if v-initial = 113 m/s with air? No air resistance air resistance

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Keep running program with different initial velocities Record initial v and time of flight

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Back to acceleration If the boy is thrown in 2 meters with a final speed of 400 m/s... from work-energy: Put in some numbers Verdict: I am pretty sure this is fake.

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Case 6: Prof Splash

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Great case for work-energy Key: what is acceleration in water? Use Work-Energy to find F-water Find acceleration

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Findings v at water = 14.55 m/s Avg acceleration in water = 347 m/s^2 This is 35.4 g’s Verdict: NASA - Approved, real

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Case 7: Giant Water slide jump

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Vertical Motion Motion not always perpendicular Unknown slide height length scale = ramp

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Other stuff Horizontal speed seems constant Accelerates in 3 seconds down the ramp - ok Risky, but possible Verdict: Fake (sometimes I get fooled also)

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Conclusion Really, all I have to say is there are more videos at http://blog.dotphys.net

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