© 2016 IOP Publishing Ltd 1 1. Introduction There are numerous publications in the literature in which the use of smartphones and speciically the iPhone (Apple Inc) have been used as an aid to teaching physics [1–12]. The iPhone is particularly suitable for mechanics experiments using the in-built acceleration sensor or accelerometer in-conjunction with the on-board data collection facility and a downloadable so-called ‘app’. Vogt and Kuhn have reviewed the use of acceleration sensors of smart- phones [13–24] and speciically investigated the acceleration of an iPhone on an inclined plane [24]. In this paper we demonstrate the novel use of an iPhone and on-board accelerometer in deter- mining the coeficient of kinetic friction, μ k of an object sliding down an inclined plane thereby making it a suitable experiment to be undertaken in the student laboratory. 2. Friction Friction is a phenomena encountered by all of us in our daily lives. In the student laboratory friction may be investigated by use of a block either stationary or sliding down an inclined plane with the coeficients of static and kinetic friction, μ s and μ k , respectively, determined. In determin- ing μ k the plane is inclined until a block resting on the plane just begins to move. The velocity of the block is then assumed to be constant with the block experiencing zero acceleration, a down the plane for an inclined angle of θ. μ k is then usually calculated from μ k = tan θ. The frustrating ‘its and starts’ of the block sliding down the plane, where the velocity is assumed to be constant, will be familiar to teachers of physics. Such its and starts involve brief periods of ‘stick’ and ‘stick- slip’ motion in which the mass spends significant periods in a state of stick (zero velocity), inter- spersed with short rapid slip periods [25]. Due to the non-steady motions involving non-rectilinear motion an error is introduced into the determina- tion of μ k as the assumption that the velocity is constant is potentially invalid. In this communi- cation the on-board accelerometer of an iPhone is used to measure acceleration of an object at P APERS Investigation of kinetic friction using an iPhone Clive Baldock 1 and Roger Johnson 2 1 Faculty of Science, Engineering and Technology, University of Tasmania, Sandy Bay, Tasmania 7005, Australia 2 Clairvaux MacKillop College, Upper Mt Gravatt, Queensland 4122, Australia E-mail: clive.baldock@utas.edu.au Abstract The iPhone is particularly suitable for mechanics experiments using the in-built acceleration sensor or accelerometer in-conjunction with the on-board data collection facility and a downloadable so-called ‘app’. In this work the iPhone has been used to investigate the acceleration due to gravity and determine the coef icient of kinetic friction, μ k of the iPhone as an object sliding down an inclined plane. This method is more accurate than that usually employed in the laboratory where the ‘its and starts’ of the block sliding down the inclined plane potentially invalidate the required assumption that the velocity is constant. In its simplest form the measurement of acceleration is required to be undertaken for only 2 angles. iopscience.org/ped 0031-9120/16/065005+6$33.00 Phys. Educ. 51 (2016) 065005 (6pp)