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question posed on yahoo's 'answers'

question posed on yahoo's 'answers' - Physics Forum

question posed on yahoo's 'answers' - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 11-24-2007, 10:58 AM
Jeremy Watts
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Default question posed on yahoo's 'answers'



This question was asked on Yahoo's 'Answers' :-

"A particle A of mass 0.5kg is suspended by a vertical string. A particle B
of mass 0.4kg is suspended from A by means of another string. A force 10N is
applied to the upper string and the particles move upwards.
a) find the tension in the string and
b) the acceleration of the system"


I've been thinking about it, and havent really convinced myself that I'm
thinking about it in the right way...

If we just considered one mass for a minute, hanging by a single string,
then with no other forces present the tension on the string would be just
'mg' right? But if there was a second force acting upwards from the top of
the string then what would the tension in the string be then? Surely it
would be the sum of 'mg' and the upward force?

Is that right??


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  #2  
Old 11-24-2007, 03:46 PM
beard
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Default question posed on yahoo's 'answers'

On Nov 24, 5:58 am, "Jeremy Watts" <[Only registered users see links. ]> wrote:

tension is the sum of the weights. Since its accelerating upward, the
10N applied doesn't add to the tension. Find the acceleration using
F=ma (10N=(0.4+0.5)a, solve for a)
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  #3  
Old 11-24-2007, 03:46 PM
beard
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Default question posed on yahoo's 'answers'

On Nov 24, 5:58 am, "Jeremy Watts" <[Only registered users see links. ]> wrote:

tension is the sum of the weights. Since its accelerating upward, the
10N applied doesn't add to the tension. Find the acceleration using
F=ma (10N=(0.4+0.5)a, solve for a)
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  #4  
Old 11-24-2007, 10:43 PM
beard
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Default question posed on yahoo's 'answers'

On Nov 24, 1:00 pm, "Herman Family"
<[Only registered users see links. ] > wrote:

no... the tension in the string caused by the masses being attached is
just their weight... this tension is enough to transmit the force, it
makes them one unit moving upwards as the new force is attached... the
acceleration should be 11.11.. m/s^2.

you think the acceleration is 0.1/0.9 or 0.11..m/s^2, for 10 newtons
applied to a mass of 0.9kg? that doesnt make sense.
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  #5  
Old 11-24-2007, 10:43 PM
beard
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Default question posed on yahoo's 'answers'

On Nov 24, 1:00 pm, "Herman Family"
<[Only registered users see links. ] > wrote:

no... the tension in the string caused by the masses being attached is
just their weight... this tension is enough to transmit the force, it
makes them one unit moving upwards as the new force is attached... the
acceleration should be 11.11.. m/s^2.

you think the acceleration is 0.1/0.9 or 0.11..m/s^2, for 10 newtons
applied to a mass of 0.9kg? that doesnt make sense.
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  #6  
Old 11-24-2007, 11:32 PM
N:dlzc D:aol T:com \(dlzc\)
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Default question posed on yahoo's 'answers'

Dear beard:

"beard" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

No, it is 10N, just as the problem statement says. The fact that
it is "9 or so" with just the effect of gravity...


Only if you forget that gravity is present.


More sense than ignoring the effect of gravity.

David A. Smith


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  #7  
Old 11-24-2007, 11:32 PM
N:dlzc D:aol T:com \(dlzc\)
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Default question posed on yahoo's 'answers'

Dear beard:

"beard" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

No, it is 10N, just as the problem statement says. The fact that
it is "9 or so" with just the effect of gravity...


Only if you forget that gravity is present.


More sense than ignoring the effect of gravity.

David A. Smith


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