The force $\vec{F}$ in Fig. $12-70$
keeps the 6.40 $\mathrm{kg}$ block and the pulleys in
equilibrium. The pulleys have negligible
mass and friction. Calculate the tension $T$ in
the upper cable. (Hint: When a cable wraps
halfway around a pulley as here, the
magnitude of its net force on the pulley is twice
the tension in the cable.)