A 4.00-kg box sits atop a 10.0-kg box on a horizontal table. The coefficient of kinetic friction between the two boxes and between the lower box and the table is 0.600, while the coefficient of static friction between these same surfaces is 0.800. A horizontal pull to the right is exerted on the lower box, as shown in the figure, and the boxes move together. What is the friction force on the UPPER box? 
Definitions:
Opportunity Cost
Opportunity cost represents the benefits an individual, investor, or business misses out on when choosing one alternative over another.
Marginal Product
Describes the additional output that is produced by using one more unit of a factor of production, holding all other factors constant.
Marginal Cost
The supplementary cost that arises when one additional unit of a product or service is produced.
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