Energy and Power of a Rotating System:

The rotational kinetic energy can be found by:

1. Summing the values for all of the particles in the system
2. Summing the values for all of the particles in the system
3. Computing the value for the system as a whole

Example 2: Energy of a Rigid Body

Instantaneous power, which is dW/dt, may be computed using . This is analogous to the Fv for systems that are in translational motion.

Newton's Second Law vs the Energy Method:

The torque version of Newton's Second Law is useful when accelerations must be found. When speeds are needed, conservation of mechanical energy is more useful.

Atwood's machine (two slightly different masses attached to a cord over a pulley) was discussed in the Newton's Laws module. At that time, we ignored any pulley effects. In my Example 3, I'll do it using the "energy balance" or conservation of energy method. (It's the same thing, remember, but with mine there are never any negative signs.) In my Example 4, I'll do the same problem using torques and forces so that you can compare the methods.

Example 3: Atwood's Machine

Example 4: Atwood's Machine, Revisited

Here's a set of four examples which cover both Newton's Laws and the Energy method.