Direct measurement of friction acting between a ballpoint pen and a paper

The friction force between a ballpoint pen and a paper has been directly measured. In the measurement method, the friction force is obtained by measuring the inertial force of a mass. A pneumatic linear bearing is used to obtain linear motion with negligible friction. The inertial force of the mass is obtained by accurately measuring the velocity of the mass by using an optical interferometer. This interferometer measures the frequency Doppler shift experienced by a laser beam that is reflected from the moving mass. Using the developed method, the friction force between a ballpoint pen and a paper was measured very accurately.