We know that energy tends to accumulate in our muscles while at rest and that it can then be expended either gradually or by one effort, but to no greater extent than the reserve force that has been accumulated. The same accumulation of energy takes place in running before taking a jump. This accumulated energy or method of gathering momentum is utilized in machines by placing a fly-wheel on the driving shaft. When such a wheel revolves, the momentum will cause it to run a long time after the power has been shut off, due to the energy stored in the fly-wheel.

Fig.41. Spring Balance.