What is a force which resists all motion

Find v(t), given acceleration a(t)=7j and initial velocity v(0)=k

An insulated 40 ft^3 rigid tank contains air at 50 psia and 120°f. a valve connected to the tank is now opened, and air is allowed to escape until the pressure inside drops to 25 psia. the air temperature during this process is kept constant by an electric resistance heater placed in the tank. determine the electrical work done during this process.

Matter is needed to transfer thermal energy bya. conductionb. convectionc. radiation d. both a & b.