Work Done by Friction

[ilovemcat]

#1 - I'm not sure if some of you read the lecture by EK regarding work done by Friction in EK, but it just totally confused my understanding of work. Based on what I read elsewhere (TPR & TBR), they mentioned nothing about needing to know the increase in internal energy of an object experiencing a frictional force over a distance, but apparently EK says it's necessary to find the work done by friction.

Work by Friction = Change in Energy of System
Fk x distance = delta PE + delta KE (lost) + delta Internal Energy (gained)

On the other hand, TPR and TBR instead just assume that the total change in mechanical energy is enough to find the amount of work done by friction.

Which is right?

#2 - Then they go on to mention that it's necessary to consider heat changes on your system. A lot of Physics equations for Work are based on the assumption that there's no heat changes involved. If there is a heat change, how does that factor in? Do we just subtract is from the total change in energy (since delta E = q + W)

#3 - Another confusion is work done by conservative forces: When do I assume that the total work is 0 J?

Can anyone help clarify this whole topic, I'm so confused lol.

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[Rabolisk]

They're both right. If you consider friction to be an external force, then the work done is the change in kinetic energy, potential energy, and internal energy of the system (which is the particle-earth system). But don't worry, for you won't have to ever worry about internal energy for the MCAT. I mean, given a standard physics problem involving a particle-like object, how are you supposed to figure out what the change in internal energy is supposed to be? On the MCAT, delta KE + delta PE (hence, delta Emech) will be enough.

For 2, you would consider heat flow, since energy transfer can be either via heat or work. But you would have to be explicitly given heat. Friction does work, not heat to the system or the object. What is heat is when the increased thermal energy of the floor or other surface is dissipated to the surroundings, but that in itself has nothing to do with the work done by friction.

For 3, total work is 0 if there is no external force (e.g. only gravity acts on an object), and if the object returns to the same position. One definition of a conservative force is that, in the absence of an external force, the net work done on a particle moving around closed path is zero. Closed path means that the object returns to the same position.

 

[plzNOCarribbean]

#1 - I'm not sure if some of you read the lecture by EK regarding work done by Friction in EK, but it just totally confused my understanding of work. Based on what I read elsewhere (TPR & TBR), they mentioned nothing about needing to know the increase in internal energy of an object experiencing a frictional force over a distance, but apparently EK says it's necessary to find the work done by friction.

Work by Friction = Change in Energy of System
Fk x distance = delta PE + delta KE (lost) + delta Internal Energy (gained)

On the other hand, TPR and TBR instead just assume that the total change in mechanical energy is enough to find the amount of work done by friction.

Which is right?

#2 - Then they go on to mention that it's necessary to consider heat changes on your system. A lot of Physics equations for Work are based on the assumption that there's no heat changes involved. If there is a heat change, how does that factor in? Do we just subtract is from the total change in energy (since delta E = q + W)

#3 - Another confusion is work done by conservative forces: When do I assume that the total work is 0 J?

Can anyone help clarify this whole topic, I'm so confused lol.

Hey, ilovemcat, how's it going? when are you going to be testing? I ended up cancelling my test date on Aug 24th, and hopefully il do okay next week? hows the studying coming along; last time I saw you post was when we were both studying for the summer mcat. Hope the all is well. Any advice on verbal?...i need a miracle

 

[ilovemcat]

They're both right. If you consider friction to be an external force, then the work done is the change in kinetic energy, potential energy, and internal energy of the system (which is the particle-earth system). But don't worry, for you won't have to ever worry about internal energy for the MCAT. I mean, given a standard physics problem involving a particle-like object, how are you supposed to figure out what the change in internal energy is supposed to be? On the MCAT, delta KE + delta PE (hence, delta Emech) will be enough.

For 2, you would consider heat flow, since energy transfer can be either via heat or work. But you would have to be explicitly given heat. Friction does work, not heat to the system or the object. What is heat is when the increased thermal energy of the floor or other surface is dissipated to the surroundings, but that in itself has nothing to do with the work done by friction.

For 3, total work is 0 if there is no external force (e.g. only gravity acts on an object), and if the object returns to the same position. One definition of a conservative force is that, in the absence of an external force, the net work done on a particle moving around closed path is zero. Closed path means that the object returns to the same position.

Ah, that makes sense. Thank you!

 

[ilovemcat]

Hey, ilovemcat, how's it going? when are you going to be testing? I ended up cancelling my test date on Aug 24th, and hopefully il do okay next week? hows the studying coming along; last time I saw you post was when we were both studying for the summer mcat. Hope the all is well. Any advice on verbal?...i need a miracle

Hey man! Good to see I'm not alone haha. I ended up taking my July exam but my computer froze during Verbal causing me to lose 3 valuable minutes and it disrupted my momentum so I ended up voiding. Didn't have a chance to re-take, so here I am doing it all over again. I forgot so much material from last summer, it's annoying. I just wanna get this thing over with.

I'm not in a great position for Verbal either, so I'm not sure if you want my advice. But I saw you exhausted a lot of verbal material so I think you'll do just fine. From my experience on the real deal, verbal wasn't that bad. They use size 14 font and most people assume it's longer than usual. But it's really not all that different from practice passages you see in EK and TPR. Just try your best and you'll do fine. Most importantly, try to stay relaxed.