## College Physics (4th Edition)

The correct answer is: B. $6.5\times 10^{-7}~N$
Since the system is in equilibrium, the torques about the fulcrum must be equal in magnitude. We can find the downward force $F$: $r_L~F = r_R~mg$ $(0.60~m)~F = (0.40~m)~(1.0\times 10^{-7}~kg)(9.80~m/s^2)$ $F = \frac{(0.40~m)~(1.0\times 10^{-7}~kg)(9.80~m/s^2)}{0.60~m}$ $F = 6.5\times 10^{-7}~N$ The correct answer is: B. $6.5\times 10^{-7}~N$