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

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

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

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

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

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

Case 1: Elephant on a trampoline

Analysis Scale = 1 trampoline

Verdict: Fake Not a constant acceleration

Case 2: Kobe number 1

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

Horizontal Motion Verdict: Could be real, but faked

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

vertical motion

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

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

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

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

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?

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

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

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

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

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

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.

Case 6: Prof Splash

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

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

Case 7: Giant Water slide jump

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

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

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

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