Question

For a machine, the output force is 20 N, and the output distance is 4 m. Which must be true about the work input? A. It equals 80 J. B. It must be greater than 80 J. C. It must be less than 80 J.

Answer 1

Work = Force*Displacement
Work = (20 N)*(4 m)
Work = (20*4) Nm
Work = 80 J

So 80 joules of work is the output. Ideally we'd expect 80 J to be the input but that's not the case. There will always be things like heat loss, leading to some level of inefficiency. So if we input some amount of work, say 100 J, then it might drop to 80 J of actual output work after the wasted energy is accounted for.

In short, you have to put more work in than the work you want out. This is why the answer is choice B) It must be greater than 80 J