Date of download: 10/16/2017 Copyright © ASME. All rights reserved. From: Improved Force Transmission of a Flexible Surgical Instrument by Combining Input Motion J. Med. Devices. 2015;9(1):011009-011009-11. doi:10.1115/1.4029418 Figure Legend: Friction force at contact point P. The unit vectors along the axial direction ea and the circumferential direction ec constitute the tangent plane at the point of contact. The resultant tangential velocity vt makes an angle φ with the unit vector ea. The resultant friction force Ft acts in the opposite direction of vt.