Tribometer

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A tribometer is an instrument that measures friction on a surface via a multitude of methods, one which is a ball rolling on the reference surface and giving a relative friction value (called ball on disc).

However, the original method is an apparatus that includes a hanging mass and a standing mass that connects the two masses with a pulley (Refer to Diagram 1). µ_{static , the coefficient of friction, which is a constant that is needed to determine the friction force and solve for the force needed to overcome it, can be measured through this apparatus.}

Diagram 1

The general equation for the friction force is the F_{f = µstaticN. N being the normal force which is equal to the weight or mg (mass x gravity).}

Thus, in this apparatus, the coefficient of friction is determined primarily by two things: the tension of the rope or string connecting the two passes through the pulley and the weight of the hanging mass.

Let’s refer to the hanging mass as m_{H and the mass on the reference surface or perhaps a flat elevated surface as mT. Since the static friction is the coefficient that determines the needed force to move a certain object, there is no acceleration on the sitting object and means that the tension of the string of the sitting object mT is equal to the friction force and the friction force is then equal to the weight of the hanging object because currently, the apparatus is not moving. Thus, Ff should equal W, or weight.}

Since F_{f is defined as µstaticN, we must substitute for the value N, the normal force. Because the sitting object is not sinking through the surface or floating up, it is safe to assume that N = mg, the weight of the object.}

Therefore, the total friction force is µ_{staticmTg = Ff. Now, if the friction force is equal to the weight of the hanging object, we can set the two values together.}

µ_{staticmTg = mHg and thus, the value for µstatic is equal to ratio of two masses, the hanging mass over the sitting mass.}

µ_{static = mHg/mTg}

In short, the tribometer measures the coefficient of static friction or the force needed to have a certain object move on a certain surface.

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