PhySP18SS6.6.17: In this case we started off adding Mechanical energy to the system for the first 7 meters, then the outside force (s) started draining energy from the system. So you added 28Joules worth of energy, but drained out 110Joules. Therefore, you KNOW the system had to have kinetic energy (it must have been moving) to begin with. I say mechanical energy since we are in a Mechanics class. I suppose it could start off as gravitational energy (energy of position) or chemical energy (tied up in chemical bonds) or nuclear energy (tied up in nucleons), but there HAD to bo some kind of Potential or kinetic energy to begin with. You can’t take $110 out of a bank account that only has $28 in it. Or . . . you can’t pour 110 gallons of water out a tank that only has 28 gallons of water in it.

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