Friction: Force and Mass

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Purpose: To study sliding friction and to determine the coefficients of static and kinetic friction. Theory: Friction is a force that opposes motion. Static friction prevents an objects being set into motion by an external force. Kinetic friction opposes the progress of a sliding object. Static friction is given by Fs< usn where N is the normal force between the object and its support surface and us is the coefficient of static friction. There-fore, the coefficient of static friction is given by Us= fs, max/ N

Where F s, max is the maxixmum force of static friction. Once the external force overcomes static friction, the object moves. Its motion is opposed by kinetic friction. The force of kinetic friction is given by f k = ukN where u k is the coefficient of kinetic friction. Therefore , the coefficient of kinetic friction is given by Uk = fk/n

A diagram of the experimental setup is shown in the figure at the right. Of interest is the friction between the block and the tabletop. An external pulling force is provided by the weight hanging on the end of a string which is adjusted to balance the force of friction. The free- body diagram of the block is shown in the figure at the left. The free body diagram of the pulling mass is shown in the figure on the right. The equations for the forces on the block in the x- and y- directions are written as EF x = 0 = T – F

EF y = 0 = N – m
Where T is the tension in the string, f is the force of friction, N is the normal force exerted on the block by the tabletop, and m is the total mass supported by the tabletop. The equation for the forces acting on the pulling mass is written as EF = 0 = mp – T

Thus, the force of friction f is equal to the pulling mass mp. The normal force N is given by N = m = mb + m added
Where mb is the mass of the block...
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