Lift Help – Which Physical Principle

Often, the Work-Energy Principle is much quicker and easier to use than direct Newton’s second law, F = ma. However, you can’t always use Work-Energy. When asking yourself whether you can use a Work-Energy approach to solving the problem, you should ask yourself a few questions:

1. Do the knowns and unknowns in the problem involve positions (displacements) and speeds? If so, then work-energy is a candidate. In contrast, time is not part of the work-energy principle. If your problem requires you to determine time, or to know time information, then work-energy is not for you.
2. Are all the forces which do work ones for which we have simple expressions for work (i.e. constant forces, gravity, and spring forces)?

If both of these are true, then your problem might be a good candidate for a work-energy approach.

If you want to see an example (not Lift, something simpler) of Work-Energy and F=ma applied to the same problem, outlining strengths and weaknesses of both approaches, I encourage you to watch the video below.

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