An 80 N box initially at rest is pulled by a horizontal rope on a horizontal table. The coefficients of kinetic and static friction between the box and the table are 0.25 and 0.5 respectively. What is the friction force on this box if the pull is 25N? A. 20 N
B. 40 N
C. 25 N
D. 0 N

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Awave travels at a speed of 5.2 m/s. if the distance between crests is 0.40 m, what is the frequency of the wave? use f=v/wavelength a. 13 hz b. 2.1 hz c. 0.077 hz d. 5.6 hz

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.