# Forces53.pdf - (ii The two surfaces will start to slip if T...

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(ii) The two surfaces will start to slip if T N m = (iii) If the two surfaces are sliding, then T N m = ± The sign in this formula must be selected so that T opposes the direction of slip. In all these formulas, m is called the `coefficient of friction’ for the two contacting materials. For most engineering contacts, 0 1 m < < . Actual values are listed below. Probably we need to explain statement (iii) in more detail. Why is there a ± ? Well, the picture shows the tangential force T acting to the right on body (1) and to the left on body (2). If (1) is stationary and (2) moves to the right, then this is the correct direction for the force and we’d use T N m = + . On the other hand, if (1) were stationary and (2) moved to the left, then we’d use T N m = − to make sure that the tangential force acts so as to oppose sliding. Friction forces at 3D contacts 3D contacts are the same, but more complicated. The tangential
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