Work, energy, and power

definitions of these and other terms related to kinetics

---

WORK involves the application of a force over a distance.

Therefore, we measure work in terms of a:

• FORCE (F) acting over a straight line DISTANCE (d).
  W=Fd

• MOMENT of force (M) acting over an arc (of length r) whose center is an axis of rotation
  W=Mr

  In either case, work is measured in units like foot*lbs. or N*m.

Because ENERGY (E) is the capacity to do WORK, we measure energy and work in the same units (foot*lbs. or N*m).

E=Fd for "straight-line" work

E=Mr for work around an axis of rotation

POWER (P) is the rate of energy generation (or absorption) over time:

P = E/t

Power's SI unit of measurement is the Watt, representing the generation or absorption of energy at the rate of 1 Joule/sec. Power's unit of measurement in the English system is the horsepower, which is equivalent to 735.7 Watts.

For translational motions:	for rotational motions:
P = Fd/t	P = Mr/t

If, in the equation for rotational motion, which includes motion around a joint axis, we substitute:

w=r/t for angular velocity (measured in degrees or radians per second), we obtain the familiar equation for JOINT POWER:

P=Mw

so that JOINT POWER is the product of:

• M (joint moment) and
• w (joint angular velocity.)

More on JOINT POWER

---

Last updated 4-22-01 ©Dave Thompson PT