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    Thread: Anyone good in physics? Well I've got a question just for YOU! Please help :)

    1. #1
      Federer's #1 Fan audreyaussie's Avatar
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      Exclamation Anyone good in physics? Well I've got a question just for YOU! Please help :)

      Two identical mass crates are pushed across the floor by two students. Crate A initially has a speed of 2.0 m/s, while crate B is initially moving with a speed of 6.0 m/s. Explain the difference in the amount of work required to bring the two crates to a stop.
      I hate it when I take off my turban and Voldemort is on my head!





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      Rational Spiritualist DrunkenArse's Avatar
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      What definitions of work do you have available to you? There's at least three that you might be using and one of them makes this super easy.

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      Federer's #1 Fan audreyaussie's Avatar
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      Aha! I think I've got it! I need to calculate their kinetic energy right? o.o
      I hate it when I take off my turban and Voldemort is on my head!





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      Rational Spiritualist DrunkenArse's Avatar
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      That's the main point yes. You know that their kinetic energy will end up being 0. So it's just a matter of what their kinetic energy is to begin with. That tells you the work to stop one crate. Then there is the matter of explaining the difference between the two levels of work.

    5. #5
      Antagonist Invader's Avatar
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      The point of the problem is to test your understanding of how kinetic energy varies with velocity. Crate B has 3 times the velocity of crate A. How much more kinetic energy does crate B have than crate A? That's going to be the difference in work required to bring each crate to a stop.

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      Rational Spiritualist DrunkenArse's Avatar
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      Quote Originally Posted by Invader View Post
      The point of the problem is to test your understanding of how kinetic energy varies with velocity. Crate B has 3 times the velocity of crate A. How much more kinetic energy does crate B have than crate A? That's going to be the difference in work required to bring each crate to a stop.
      Hey, back off! Can't you read the title? The question was just for ME! Quit Invading my thread dude. Oh....wait a minute.

    7. #7
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      I would say it is the same for each, simply stop pushing. I do not recall work being defined by the removal of effort.

      This seems to be one of those questions for people who think too much but accomplish too little with it.
      Last edited by Philosopher8659; 11-12-2010 at 06:10 PM.

    8. #8
      Rational Spiritualist DrunkenArse's Avatar
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      Quote Originally Posted by Philosopher8659 View Post
      I would say it is the same for each, simply stop pushing. I do not recall work being defined by the removal of effort.

      This seems to be one of those questions for people who think too much but accomplish too little with it.


      I'm sorry, haven't you been banned from helping people with technical homework yet? run along now and go square a circle. And unlike my last comment, I'm not joking.
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      Pout.

    10. #10
      Antagonist Invader's Avatar
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      Quote Originally Posted by Philosopher8659 View Post
      I would say it is the same for each, simply stop pushing. I do not recall work being defined by the removal of effort.
      Work is defined as the amount of energy transfered to a system through forces that act on that system in some distance. Being that all bodies in motion will remain in the same, uniform motion unless external forces are enacted upon them, what do you think causes, say, a crate moving over a flat surface to suddenly slow down? The magical phenomenon at work here (no pun intended) is called friction. This is why you can "simply stop pushing". There is still a force being exerted against the motion of the crate in some distance. Energy is being transferred as kinetic energy to the Earth, and as thermal energy to the contact materials where the crate and ground touch. Differences in velocity are exponentially proportional to the kinetic energy of the system, and differences in mass are directly proportional.

      The purpose of the question is to build a fundamental understanding of velocity's relationship to kinetic energy. That is all.

      Quote Originally Posted by PhilosopherStoned View Post
      Can't you read the title? The question was just for ME!
      I see wut you implied thar.
      Last edited by Invader; 11-12-2010 at 10:51 PM.

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      Quote Originally Posted by Invader View Post
      Work is defined as the amount of energy transfered to a system through forces that act on that system in some distance. Being that all bodies in motion will remain in the same, uniform motion unless external forces are enacted upon them, what do you think causes, say, a crate moving over a flat surface to suddenly slow down? The magical phenomenon at work here (no pun intended) is called friction. This is why you can "simply stop pushing". There is still a force being exerted against the motion of the crate in some distance. Energy is being transferred as kinetic energy to the Earth, and as thermal energy to the contact materials where the crate and ground touch. Differences in velocity are exponentially proportional to the kinetic energy of the system, and differences in mass are directly proportional.

      The purpose of the question is to build a fundamental understanding of velocity's relationship to kinetic energy. That is all.
      I think you need to read the physics book again.

    12. #12
      Antagonist Invader's Avatar
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      Please point out the specific statements you disagree with and post the corrections to them. Humor me.

      The mathematical definition for work, W = Fd = m(v2-v1)2, is not contrary to anything I've said.

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      Is work the same as the removal of velocity? Work is always positive.

    14. #14
      Rational Spiritualist DrunkenArse's Avatar
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      Quote Originally Posted by Invader View Post

      I see wut you implied thar.
      That the title of the thread promised me that because I'm good at physics the question was just for me and made no mention of other people that might be good a physics (or squaring circles)?


      Quote Originally Posted by Philosopher8659 View Post
      This seems to be one of those questions for people who think too much but accomplish too little with it.
      Yes. Putting shit in space, airplanes, skyscrapers, cellphones, computers, etc. is "too little". You've made your "point", now please run along.


      Quote Originally Posted by Philosopher8659 View Post
      I think you need to read the physics book again.
      I think you need to read the physics book to begin with. And FYI, I'm not talking about Aristotle.

      Quote Originally Posted by Philosopher8659 View Post
      Is work the same as the removal of velocity? Work is always positive.
      As defined with Invader's typo, yes it is always positive (so long as v is real). Unfortunately for your assertion, it's a typo.

      W = m(v22-v12)

      As you can see, this can clearly be negative. It is equivalent to a change in kinetic energy.
      Last edited by PhilosopherStoned; 11-12-2010 at 11:26 PM.

    15. #15
      Antagonist Invader's Avatar
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      Quote Originally Posted by PhilosopherStoned View Post
      That the title of the thread promised me that because I'm good at physics the question was just for me and made no mention of other people that might be good a physics (or squaring circles)?
      Precisely my good fellow!

      Quote Originally Posted by Philosopher8659 View Post
      Is work the same as the removal of velocity? Work is always positive.
      Check it out, we'll define our terms:

      Work is the transfer of energy and is expressed in SI units as "Joules".
      Velocity is a vector quantity expressed, in SI units, as meters per second, or m/s. It is the distance being covered per second in some direction.

      Now, if you asked me again whether or not work is the same as removal of velocity (removal of velocity is change in velocity, dv), what do you think my answer would be? The units they are measured in are not even the same.

      Also, work can be negative if energy is being removed from the system in question. Elementary physics, my dear.
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      Federer's #1 Fan audreyaussie's Avatar
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      Wow! So many replies Anyways, you've all been a BIG help.

      Thanks a bunch!

      I hate it when I take off my turban and Voldemort is on my head!





      <3 Love you Voldy!

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      Member nina's Avatar
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      mmm Invader...talk physics to me

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