If the scale reads 650N, then the mass of whoever it is standing on the scale is
(weight) / (gravity) = (650N) / (9.8 m/s²) = 66.3 kilograms .
It's not MY mass, even if I'm the one standing on the scale.
If I stand on a scale and it reads 650 N, the scale is broken.
It would last as long as the applied force continued, or until the accelerating object hit something.
Efficiency = (energy that does the job) / (total energy used)
= (45 J) / (120J)
I think you can handle the division.
