Energy used on a rowing machine? On a rowing machine, the energy / power applied to the handle is measured by the machine but the energy needed to move myself up and down the slide (forwards and backwards) is not measured. I am trying to work out the energy cost (joules) of moving my center of mass through a stroke (my mass is still at start, accelerate and move backwards, decelerate to stop and reverse direction back to starting point).
Mass = 118 kg, distance moved by center of mass = 89 cm in each direction. Time (don't know if this is relevant) = 3 s for whole stroke - so 1.5 s for each direction of travel.
 A: Youre moving horizontally, so the distance per se doesn’t matter because there is no external resistance to that motion.
But decelerating yourself, then accelerating yourself… those take energy.
We are not allowed to do it for you, but I would start with figuring out what rate of acceleration you need based on distance traveled and time to do it. In other words if you’re at the farthest point back then you’re not moving you have to accelerate to a speed then move a distance and stop. Even though the forces are in different directions, because you’re a person they don’t cancel out you don’t store up forces because they’re in different directions. So every force involved in accelerating and decelerating is an expenditure of energy, and the amount of energy expended can be determined by $\int Fdx$ as long as you don’t return in a negative values to yourself as energy.
If you know how far a mass goes, and in what time, at constant acceleration, starting at rest, what is that acceleration? Do you know the equation? And then from $F=ma$, what force was needed? How do you get energy expended by force and distance?
