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Thread: Input Work vs Input Force

  1. #1 Input Work vs Input Force 
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    Hi Folks,

    I am having a devil of a time understand the concept of input work vs input force (relating to machines).

    OK -- I get that you push down a car's pedal and the car moves. Your force is vastly increased by the car's engine. The input force is much much less than the output force. The car has a mechanical advantage of some factor.

    But I don't understand how this isn't the same terminology as work. If my input work is pushing the pedal, the car still moves the same amount. The amount of work done by the car is a huge amount more than mine. Sure, my work is the same, but the ouput work is much more than the input work.

    What am I missing here?!?!!

    Thanks!
    WT


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  3. #2  
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    The energy to move the car is coming from the gasoline that you are burning, not from your force on the pedal.


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  4. #3  
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    Now do we say that the car itself is doing work? Because then I think I get it, but it's still fuzzy.

    My work is the action of my foot on the pedal. But the mechanics of the engine are doing the work to make things move. Does that basically mean that any time I use a machine, I am relinquishing my ownership of the work? If so, at what point do I relinquish it?

    I'm also looking at a sentence in my texbook that says "machines make work easier, but they don't decrease the amount of work that I do."

    How do I know that I'm doing the work and not relying on something else?
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  5. #4 Re: Input Work vs Input Force 
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    Quote Originally Posted by WriterTyper
    Hi Folks,

    I am having a devil of a time understand the concept of input work vs input force (relating to machines).
    Work is energy, force x distance over which the force is exerted. Force is just force.

    Quote Originally Posted by WriterTyper
    OK -- I get that you push down a car's pedal and the car moves. Your force is vastly increased by the car's engine. The input force is much much less than the output force. The car has a mechanical advantage of some factor.
    Nope.

    In a car you are simply controlling the release of chemical energy stored in the molecules of gasoline, in an engine governed by the laws of thermodynamics.

    This has little to do with force.

    Quote Originally Posted by WriterTyper
    But I don't understand how this isn't the same terminology as work. If my input work is pushing the pedal, the car still moves the same amount. The amount of work done by the car is a huge amount more than mine. Sure, my work is the same, but the ouput work is much more than the input work.

    What am I missing here?!?!!

    Thanks!
    WT
    You are misunderstanding the term "force". You need to read a chapter on mechanics in an introductory physics text. Any text will do.
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  6. #5 Re: Input Work vs Input Force 
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    Quote Originally Posted by DrRocket
    You are misunderstanding the term "force". You need to read a chapter on mechanics in an introductory physics text. Any text will do.
    Ha, that is exactly what I'm reading! And that's what I don't understand!

    I'm being introduced to the concept of machines with the two sentences:

    1. Machines make work easier.
    2. Work is done when a force causes an object to move in the same direction that the force is applied
    3. The output work of a machine cannot be greater than the input work.

    This makes me think that two opposite things are being said. First, that a machine will make it easier for me to exert a force, but also that that force won't be greater than what I've exerted. How can something not be greater, yet be easier?
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  7. #6 Re: Input Work vs Input Force 
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    Quote Originally Posted by WriterTyper
    Hi Folks,

    I am having a devil of a time understand the concept of input work vs input force (relating to machines).

    OK -- I get that you push down a car's pedal and the car moves. Your force is vastly increased by the car's engine. The input force is much much less than the output force. The car has a mechanical advantage of some factor.

    But I don't understand how this isn't the same terminology as work. If my input work is pushing the pedal, the car still moves the same amount. The amount of work done by the car is a huge amount more than mine. Sure, my work is the same, but the ouput work is much more than the input work.

    What am I missing here?!?!!

    Thanks!
    WT
    Force is a measure of the acceleration applied to a mass. Work is a measure of force applied over a distance.
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  8. #7 Re: Input Work vs Input Force 
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    Quote Originally Posted by WriterTyper
    This makes me think that two opposite things are being said. First, that a machine will make it easier for me to exert a force, but also that that force won't be greater than what I've exerted. How can something not be greater, yet be easier?
    This is wrong. Read the first sentence in my earlier post.
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  9. #8 Re: Input Work vs Input Force 
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    Quote Originally Posted by WriterTyper
    Quote Originally Posted by DrRocket
    You are misunderstanding the term "force". You need to read a chapter on mechanics in an introductory physics text. Any text will do.
    Ha, that is exactly what I'm reading! And that's what I don't understand!

    I'm being introduced to the concept of machines with the two sentences:

    1. Machines make work easier.
    2. Work is done when a force causes an object to move in the same direction that the force is applied
    3. The output work of a machine cannot be greater than the input work.

    This makes me think that two opposite things are being said. First, that a machine will make it easier for me to exert a force, but also that that force won't be greater than what I've exerted. How can something not be greater, yet be easier?
    Work is force times distance.

    So let's look at a simple lever:

    You have a 10 pound weight on one end of the lever and this end is 1/10 the distance from the fulcrum as the other end.

    I can lift the ten pound weight by pushing down on the other end and it only takes 1 lb of force to due so. However, for every foot I move the end downward, the weight only rises 1/10 of a foot.

    Thus the lever makes it easier to lift the weight by reducing the amount of force I need to apply, but the total amount of work I do, applying 1 lb of force over a distance of 1ft, equal the same amount of work I get out at the weight's end by lifting 10 lbs through a distance of 1/10 ft.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  10. #9 Re: Input Work vs Input Force 
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    Quote Originally Posted by Janus
    Quote Originally Posted by WriterTyper
    Quote Originally Posted by DrRocket
    You are misunderstanding the term "force". You need to read a chapter on mechanics in an introductory physics text. Any text will do.
    Ha, that is exactly what I'm reading! And that's what I don't understand!

    I'm being introduced to the concept of machines with the two sentences:

    1. Machines make work easier.
    2. Work is done when a force causes an object to move in the same direction that the force is applied
    3. The output work of a machine cannot be greater than the input work.

    This makes me think that two opposite things are being said. First, that a machine will make it easier for me to exert a force, but also that that force won't be greater than what I've exerted. How can something not be greater, yet be easier?
    Work is force times distance.

    So let's look at a simple lever:

    You have a 10 pound weight on one end of the lever and this end is 1/10 the distance from the fulcrum as the other end.

    I can lift the ten pound weight by pushing down on the other end and it only takes 1 lb of force to due so. However, for every foot I move the end downward, the weight only rises 1/10 of a foot.

    Thus the lever makes it easier to lift the weight by reducing the amount of force I need to apply, but the total amount of work I do, applying 1 lb of force over a distance of 1ft, equal the same amount of work I get out at the weight's end by lifting 10 lbs through a distance of 1/10 ft.
    And if you pish on the other end it takes 100 lb force to move the weight, but the weight moves 10 times as far as do you. But the lever is still a machine, even though it makes it harder to lift the weight. Exactly this principle is used in the throwing stick for an atlatl -- it trades force for speed.
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  11. #10  
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    Ok, I think I may have come up with a metaphor to help me understand (it's how I understand the best).

    If I pour 5 liters of water into a bucket, then I make six holes, only 5 liters of water can come out of this bucket.

    But if I take all day, drip by drip, to fill up the bucket, then I make six holes, the water will spray out much more quickly than it took me to fill the bucket.

    In other words, the amount of energy I put into the system remains constant, but the force in which the energy can travel will increase because of the system's construction.

    Is that a good metaphor?
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  12. #11  
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    Quote Originally Posted by WriterTyper
    In other words, the amount of energy I put into the system remains constant, but the force in which the energy can travel will increase because of the system's construction.
    bold added

    This makes no sense, and indicates that youm still don't understand either "force" or "energy".

    Better re-read the earlier posts and that chapter.
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  13. #12  
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    I see. You are correct. Could I then modify the sentence to say:

    "The energy I put into the system always remains constant. However, a machine increases the force applied to the object."?
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  14. #13  
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    Quote Originally Posted by WriterTyper
    I see. You are correct. Could I then modify the sentence to say:

    "The energy I put into the system always remains constant. However, a machine increases the force applied to the object."?
    no

    Go read again. See in particular my comment to the post by Janus.
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