The discussion revolves around the concepts of static friction and work done on a block placed on a sheet of paper being pulled. Participants explore the nature of static friction, its classification as conservative or non-conservative, and the implications of work done in different reference frames.
The discussion is active, with participants sharing insights and questioning assumptions about the nature of static and kinetic friction. Some guidance has been offered regarding the definitions of conservative and dissipative forces, but no consensus has been reached on the classification of static friction.
Participants note the importance of reference frames in understanding work done and the behavior of forces. There is an ongoing debate about the terminology used to describe non-conservative forces and their effects on mechanical energy.
PhanthomJay said:That is correct, as long as the block is accelerating with respect to the ground. The work done by friction by the paper on the block is positive work with respect to the ground. Work like energy is relative. From the reference frame of the paper, friction does no work on the block , because there is no motion with respect to the paper.
For a force to be conservative, the work done has to be zero over every possible closed loop. It is not sufficient that the work done is zero over some particular closed path.Caio Graco said:OK thank you. It is easy to see that if the paper is pulled into the starting position, bringing the block to the starting position also (at rest), the total work on the block will be zero. Does this mean that the static friction force is conservative?
jbriggs444 said:For a force to be conservative, the work done has to be zero over every possible closed loop. It is not sufficient that the work done is zero over some particular closed path.
Yes. If the block started at rest, moved with the paper throughout and ended with some non-zero velocity, that would demonstrate that static friction is not a conservative force. [Similarly, if it had started in motion and ended at rest. Or started in motion and ended with a different speed].Caio Graco said:However, if the initial position of the block has some velocity, this already shows a case in which the work is non-zero, which indicates that the static friction force is non-conservative. I believe this is what you call my attention.
I am not comfortable using the term "dissipative" for static friction. The third law force pair involved with static friction never dissipates any mechanical energy into heat. So although it is not "conservative", I would not call static friction "dissipative".So can I say that the static friction force is dissipative? It is?
Your hand (presuming it's you doing the pulling) is exerting a force on the sheet.Caio Graco said:I can not see another force doing work other than the static frictional force on the block.
Mister T said:Your hand (presuming it's you doing the pulling) is exerting a force on the sheet.
By the way, whatever energy you expend doing this will cause the temperatures of the surfaces that do slide against each other to warm up.
No, the force isn't causing mechanical energy to be lost and converted to thermal energy. A dissipative force is non-conservative, but not all non-conservative forces are dissipative.Caio Graco said:So can I say that the static friction force is dissipative?
If you search on Google, you will see that nonconservative and dissipative forces are given as synonyms. Just as noted @ jbriggs444 in post # 6:vela said:...but not all non-conservative forces are dissipative.
One of our members, @sophiecentaur has a signature line that I've always liked: "Classification is the enemy of understanding". The important thing is knowing how something behaves, not what label can be slapped on it.Caio Graco said:Conclusion: The force of kinetic friction, even if it is non-conservative, can be dissipative or motor. This is it?
Strange how this confusion exists when, in other circumstances, people are quite happy to talk in terms of Efficiency in machines when the Work OUT comes from the Work IN due to slipping friction in a mechanism. (Clutches, dragsters etc.) It's all those over simplified 'definitions' that you find in textbooks that are to blame.jbriggs444 said:One of our members, @sophiecentaur has a signature line that I've always liked: "Classification is the enemy of understanding". The important thing is knowing how something behaves, not what label can be slapped on it.
Yes, if one drops a piece of wood onto a belt sander and watches it fly away, and if one draws system boundaries so that the piece of wood is inside the system and the moving belt is outside, the force of kinetic friction will have added mechanical energy to the system.
The label "dissipative" on the force does not tell you this. The fact that an external force was applied to a block that was (on average during the application of the force) moving tells you this.