andieje
asked on
basic physics - pulley question
Hi
This is the question:
A bucket is being raised by a single pulley by a height of 4m
O\
| \
| \
B F B=bucket, F=force
The weight of the bucket is 200N. The force exerted by the worker is 210N
How much work is done by the worker?
To calculate work all i know is force * distance travelled = 210 * 4m = 840J which is the answer in the book
Why is F bigger than the weight of the bucket or why is work done on bucket < work done by worker?
No answer in book for this (it only gives numerical answers)
I presume some of the energy is lost as friction. However the worker isn't pulling straight down to raise the bucket. Does this have anything to do with it? The book hasn't covered anything like this yet but i know from experience you can change the force needed by the angle you pull at.
thanks
This is the question:
A bucket is being raised by a single pulley by a height of 4m
O\
| \
| \
B F B=bucket, F=force
The weight of the bucket is 200N. The force exerted by the worker is 210N
How much work is done by the worker?
To calculate work all i know is force * distance travelled = 210 * 4m = 840J which is the answer in the book
Why is F bigger than the weight of the bucket or why is work done on bucket < work done by worker?
No answer in book for this (it only gives numerical answers)
I presume some of the energy is lost as friction. However the worker isn't pulling straight down to raise the bucket. Does this have anything to do with it? The book hasn't covered anything like this yet but i know from experience you can change the force needed by the angle you pull at.
thanks
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> If the applied force were equal to the weight of the bucket, it would not move.
It would not accelerate, but it would continue moving if it was already moving.
> The extra force may be required to overcome friction or it could accelerate the bucket.
or the weight of the rope...
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>> Then you can drop your force down to the bucket's weight.
Just to be clearer (because I wasn't) I am still talking about a frictionless system.
Just to be clearer (because I wasn't) I am still talking about a frictionless system.
ASKER
Hi
If the angle has nothing to do with it (and I'm not disagreeing i'm just curious) why when i use a piece of equipment a bit like this at the gym is it easier to pull the weight at a less steep angle (e.g more outwards rather than downwards). Perhaps that's just to do with how muscles work rather than the actual force needed
If the angle has nothing to do with it (and I'm not disagreeing i'm just curious) why when i use a piece of equipment a bit like this at the gym is it easier to pull the weight at a less steep angle (e.g more outwards rather than downwards). Perhaps that's just to do with how muscles work rather than the actual force needed
When you pull down, you get to use more of your weight more efficiently.
But the tension in the rope required to move the weight does not depend on the angle.
But the tension in the rope required to move the weight does not depend on the angle.
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ASKER
many thanks everyone
"This is the question:
A bucket is being raised by a single pulley by a height of 4m
How much work is done by the worker?
To calculate work all i know is force * distance travelled = 210 * 4m = 840J which is the answer in the book"
And that is all you need to know for this problem.
"Why is F bigger than the weight of the bucket "
Because that is the number given by the author
"why is work done on bucket < work done by worker?"
The work on on the bucket is not less than the work done by the worker. The bucket has increased its KE as well as its PE.
Problems of this type in books of this type almost always assume frictionless pulleys unless friction is explicitly stated.
In this case the weight of the bucket is entirely extraneous.
-
Just for my curiosity, I would appreciate some information about the book you are using. Is it a textbook.? for what level? what is its reputation?
A bucket is being raised by a single pulley by a height of 4m
How much work is done by the worker?
To calculate work all i know is force * distance travelled = 210 * 4m = 840J which is the answer in the book"
And that is all you need to know for this problem.
"Why is F bigger than the weight of the bucket "
Because that is the number given by the author
"why is work done on bucket < work done by worker?"
The work on on the bucket is not less than the work done by the worker. The bucket has increased its KE as well as its PE.
Problems of this type in books of this type almost always assume frictionless pulleys unless friction is explicitly stated.
In this case the weight of the bucket is entirely extraneous.
-
Just for my curiosity, I would appreciate some information about the book you are using. Is it a textbook.? for what level? what is its reputation?
If your book is a practice book for an exam (especially a multiple choice exam, no essay or qualifying assumptions allowed), then aburr is right on. The work done by the person is his 410N over 4 m; and the effective work done on the bucket is 400N over 4m. In that case all our other answers based on various assumptions, e.g., friction or not, constant speed or accelerating, whether or not the speed of the bucket was 0 or not at the beginning and end of this 4m journey do provide you with additional insight into the laws of physics; but in a sense are throwing you off track if this is for a timed test.
In a timed test, you already have gone through and hopefully mastered the insightful issues, and then can focus on the question, and realize that sometimes, assumptions about the system do not need to be made.
If that is the nature of the book, then I would recommend using a true learning book first, and then go over practice exam questions second.
>> "why is work done on bucket < work done by worker?"
>>>>The work on on the bucket is not less than the work done by the worker. The bucket has increased its KE as well as its PE.
As aburr says "Problems of this type in books of this type almost always assume frictionless pulleys unless friction is explicitly stated". Maybe that is true - it's been a long time since I've read this book; my own recollection is that the assumptions were either not ambiguous, or we had to state our assumptions explicitly; and in some problems, the stated assumptions had to be reasonable and non-trivial. You should review your book about what it's "tone" is - does it expect assumptions or not.
In this latter case with a constant force on a frictionless system, then you probably understand that the bucket is still moving upwards at the 4m mark with the same acceleration as it had at the beginning of its journey (unless you make the assumption that the person stops pulling at the 4m mark, in which case the bucket continues upwards while decelerating and then drops back down to the 4m mark).
In a timed test, you already have gone through and hopefully mastered the insightful issues, and then can focus on the question, and realize that sometimes, assumptions about the system do not need to be made.
If that is the nature of the book, then I would recommend using a true learning book first, and then go over practice exam questions second.
>> "why is work done on bucket < work done by worker?"
>>>>The work on on the bucket is not less than the work done by the worker. The bucket has increased its KE as well as its PE.
As aburr says "Problems of this type in books of this type almost always assume frictionless pulleys unless friction is explicitly stated". Maybe that is true - it's been a long time since I've read this book; my own recollection is that the assumptions were either not ambiguous, or we had to state our assumptions explicitly; and in some problems, the stated assumptions had to be reasonable and non-trivial. You should review your book about what it's "tone" is - does it expect assumptions or not.
In this latter case with a constant force on a frictionless system, then you probably understand that the bucket is still moving upwards at the 4m mark with the same acceleration as it had at the beginning of its journey (unless you make the assumption that the person stops pulling at the 4m mark, in which case the bucket continues upwards while decelerating and then drops back down to the 4m mark).
>> The work done by the person is his 410N over 4 m; and the effective work done on the bucket is 400N over 4m.
Sorry, just flip-flopped about the "assumptions" again! :(
Going with frictionless system, and constant 410N force, then the force on the bucket is 410N and the bucket's delta (PE + KE) over the 4m will be the work done on it.
Sorry, just flip-flopped about the "assumptions" again! :(
Going with frictionless system, and constant 410N force, then the force on the bucket is 410N and the bucket's delta (PE + KE) over the 4m will be the work done on it.
Clarification: I did not explicitly say this in my answer but I want to make sure it is clear. The total work was force applied to the system times distance over which that force was applied. 840J. No other work went into the system. total answer.
Yes, that work went into and was stored by the system in various ways (PE and KE if frictionless, PE, KE, and Heat if not frictionless) While intellectually interesting...answering that was not part of the book question and we are correct to just stop at that point in the answer. (Although it is Grand Fun to look into the details as it were and figure out where everything went.) In the end we did not even really need to know the force on the bucket, its mass or even initial velocity. Those would just affect how much of the work would go into PE, how much into KE, etc. Aburr was quite right and in the end had the most elegant solution. Although maybe not the most Fun solution :-) bye all
Yes, that work went into and was stored by the system in various ways (PE and KE if frictionless, PE, KE, and Heat if not frictionless) While intellectually interesting...answering that was not part of the book question and we are correct to just stop at that point in the answer. (Although it is Grand Fun to look into the details as it were and figure out where everything went.) In the end we did not even really need to know the force on the bucket, its mass or even initial velocity. Those would just affect how much of the work would go into PE, how much into KE, etc. Aburr was quite right and in the end had the most elegant solution. Although maybe not the most Fun solution :-) bye all
correction: >> 410N over 4 m ; 400N over 4m
I meant 210 and 200 over 4m - a sort of a typo
I meant 210 and 200 over 4m - a sort of a typo
It could have something to do with increasing friction, but in an idealized world of frictionless pulleys and massless springs, the change in direction has no effect on the work done.