A heavy box of mass 50 kg is moving on a horizontal surface. If the coefficient of kinetic friction between the box and the horizontal surface is 0.3, then the force of kinetic friction is:
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The force of kinetic friction is given by \( F_k = \mu_k \cdot N \), where \( N \) is the normal force and \( \mu_k \) is the coefficient of kinetic friction.