In a short film called Kung Fury, the main hero uses a person as a surfing board. We investigated the force he would need to push himself forward based on the kinetic friction and drag force. The force ranges between -335.24 ± 6.87 N and -980.74 ± 6.87 N

Harris, Ben G.S.

Harrison, Zoe

Holesinsky, Tomas

Mandhai, Soheb

English

University of Leicester Open Journals

Journal of Physics Special TopicsAn undergraduate physics journalP2 6 Surfin’ FuryT. Holesinsky, Z. Harrison, S. Mandhai, B. G. S. HarrisDepartment of Physics and Astronomy, University of Leicester, Leicester, LE1 7RHDecember 1, 2016AbstractIn a short film called Kung Fury, the main hero uses a person as a surfing board. We investigate theforce he would need to push himself forward based on the kinetic friction and drag force. We additionallyestimate the maximum friction he would be able to overcome. The force ranges between 335.24± 6.87 Nand 980.74± 6.87 N. The value for coefficient of friction is estimated as 0.72.IntroductionIn a spoof film of 80s culture, Kung Fury, themain protagonist of the same name fights anarmy of Nazis [1]. In the fighting scene he usesone of the Nazi soldiers as a surf board. We in-vestigate the force he would have to exert to pro-pel himself at average skateboard speed as thesituation resembles both skateboard and surf-board. We also approximate the maximum co-efficient of kinetic friction he would be able toovercome.TheoryFigure 1: The free body diagram of forces acting onKung Fury while surfing.We start by identifying all forces acting onKung Fury and his “surfboard” as seen in Figure1. The two drag forces come from Kung Fury asa standing person and the soldier in supine po-sition. The drag force can be calculated usingfollowing equationFD =12CDAρvi2 (1)where CD is drag coefficient, A is the cross sec-tional area, ρ is the density of air, and vi is thespeed of the object [2]. The drag coefficient fora standing person is 1.2 and for supine person0.2 [3]. The cross sectional area for Kung Furywas taken as a rectangle of dimensions 1.74 mand 0.65 m [6]. As the soldier is lying his crosssectional area was also assumed as a rectangle ofwidth 0.65 m and we assumed height of 0.1 m.The average speed achieved on a skateboard isabout 6.7 ms−1 [4]. The density of air at 1 atmis 1.225 kgm−3.We assumed that the motion starts with initialvelocity. Thus we only consider the kinetic fric-tion coefficient. The value of kinetic friction co-efficient, µk, between leather and steel was foundto be 0.25 [5]. This value was the most compa-P2 6 Surfin’ Fury, December 1, 2016rable one found.To determine the force Kung Fury has to over-come to be able to maintain his speed, we usedthe following equationFf + FD1 + FD2 = Fp (2)where Ff = µkmg is the frictional force, FD1and FD2 are the standing and prone drag forces,and Fp is the force necessary for propelling. Weassumed the mass of each person to be 70 kg. Sototal mass is 140 kg.DiscussionFigure 2: The relationship between the kinetic frictioncoefficient and the propulsion force. The point indicatedin red is for the leather on metal coefficient of kineticfriction. The blue point signifies the maximum coefficientof kinetic friction Kung Fury would surpass. All the datapoints have error value of ± 6.87 N.The force necessary to thrust Kung Fury at aconstant speed was found to beFp = 335.24 ± 6.87 NWe have also decided to investigate what maximvalue of coefficient of kinetic friction would allowthe motion to be unhindered and produced Fig-ure 2. In Figure 2 we assumed that Kung Furyshould be able to push with force of Fp = 980.74± 6.87 N.This is our assumption of maximum force hecould apply with one leg based on his body mass[7]. The blue point provides the maximum valueof kinetic friction coefficient at which he wouldbe able to remain at constant speed ofµk = 0.72The values of the drag forces are as FD1 = 5.57N and FD2 = 0.05 ± 0.005 N. These values arevery low and affect the motion minimally due tothe low speed.The errors arise from the assumptions in thecross sectional area and have the same value onall data points (± 6.87 N).ConclusionBased on our model it appears to be possiblefor Kung Fury to use a human being as a surfboard. The force of 335.24± 6.87 N provides anidea of how much force he is applying in the film.Furthermore, the force value of 980.74 ± 6.87 Nbased on our assumption of his physical capa-bilities sets a maximum value for kinetic frictioncoefficient of 0.72 that would allow surfing to oc-cur.References[1] http://www.kungfury.com/(Accessed 27/11/2016)[2] Tipler, P. A. Tipler and G. Mosca, PhysicsFor Scientists and Engineers With ModernPhysics, (W. H. Freeman and Company, NewYork, 2008)[3] https://goo.gl/VtxBqS(Accessed 16/11/2016)[4] https://goo.gl/m0rsS5(Accessed 16/11/2016)[5] https://goo.gl/yfE6XL(Accessed 16/11/2016)[6] https://goo.gl/Gktydi(Accessed 14/10/2016)[7] https://goo.gl/ddRh72(Accessed 16/11/2016)

P2_6 Surfin' Fury

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P2_6 Surfin' Fury

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