Why Ignore Tension In Pulley and Work Energy Problems?

[LickMyEyeball]

A pulley and block system is a favorite type of system for analysis when considering work-energy problems in beginning dynamics.

It's commonly recommended that the best approach to the system is to write an equation for the whole system, while ignoring the work done by cables/ropes and focusing on the blocks themselves.

What is the property of rope and pulley systems that allows the analysis to ignore them in this case? I'm just curious and am looking for insight.

Thank you for your time.

 

[alan2]

It would unnecessarily complicate the analysis when the point is to learn to solve simple problems. It is true that some work is required to rotate the pulleys, ropes are not massless, etc., but in real world systems those effects are usually small compared to whatever work is required to accomplish the task as hand, such as lifting a heavy mass, and so can be ignored.

 

[LickMyEyeball]

It would unnecessarily complicate the analysis when the point is to learn to solve simple problems. It is true that some work is required to rotate the pulleys, ropes are not massless, etc., but in real world systems those effects are usually small compared to whatever work is required to accomplish the task as hand, such as lifting a heavy mass, and so can be ignored.

Thanks for your reply. So I understand that it's more a question of differing orders of magnitude, as opposed to some intrinsic property of pulley systems, say, always performing work functions that equal out to zero, for example.

 

[alan2]

Yes. It is akin to the perfectly frictionless surface.

 

[CWatters]

Thanks for your reply. So I understand that it's more a question of differing orders of magnitude, as opposed to some intrinsic property of pulley systems, say, always performing work functions that equal out to zero, for example.

Yes the former.

In the real world friction in pulleys and gears can't always be ignored. Particularly if you have a high ratio step-up arrangement where any friction in the output is magnified by the gear ratio to increase the required input torque considerably.