A person standing on a spring balance inside a stationary lift measures 60 kg. The weight of that person if the lift descends with uniform downward acceleration of $1.8 \mathrm {~m} \mathrm {~s} ^ { - 2 }$ will be $\_\_\_\_$ $\mathrm { N } . \left[ g = 10 \mathrm {~m} \mathrm {~s} ^ { - 2 } \right]$