A custodian pushes a 138 kg box across a level floor. The box moves at a constant velocity. The custodian pushes horizontally on the box with a force of 853 Newtons, what must be the friction force experienced by the box in Newtons?

Consider a 1000 w iron whose base plate is made of 0.5 cm thick aluminum alloy 2024-t6 (ρ = 2770 kg/m3 and cp = 875 j/kg°c). the base plate has a surface area of 0.03 m2. initially, the iron is in thermal equilibrium with the ambient air at 22°c. assuming 90% of the heat generated in the resistance wires is transferred to the plate, determine the minimum time needed for the plate temperature to reach 200°c.