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      05-30-2007, 11:58 PM   #1
BMWPower06
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Don't Hate Me. More Physics

Ok im a real dumbass when it comes to this shit, yet it seems to come naturally for most of the members on here.

Problem 1:
A window washer pulls herself upward using the bucket-pulley apparatus shown in figure below.

How hard must she pull downward to raise herself slowly at constant speed? The mass of the person plus the bucket is 65.2 kg.

And, if she increases this force by 10.4 percent, what will her acceleration be?


Problem 2:
A 3.15 kg block is pushed along the ceiling with a constant applied force of F = 87.4 N that acts at an angle of alpha = 58.9° with the horizontal, as seen in the figure below.

The block accelerates to the right at 5.49 m/s2. Determine the coefficient of kinetic friction between block and ceiling.


TIA guys
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      05-31-2007, 12:02 AM   #2
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      05-31-2007, 12:04 AM   #3
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Quote:
Originally Posted by Fartpimpson View Post
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      05-31-2007, 02:06 AM   #4
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bump. where are all the physics guys tonite?
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      05-31-2007, 02:14 AM   #5
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hahaha out drinking

come back in an hour or twwo and they might show up
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      05-31-2007, 02:26 AM   #6
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Sorry I don't have your answer but this may help:

Problem 1
force = mass x acceleration
Since speed is constant, acceleration will be gravity.
Since acceleration is gravity, the force is justified by the weight
force = 65.2 kg (9.8 m/s2)

at 10.4% more force:
just find acceleration if force is increased by 10.4%
acceleration = force(10.4%)/65.2
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      05-31-2007, 02:28 AM   #7
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Quote:
Originally Posted by Driv3r View Post
hahaha out drinking

come back in an hour or twwo and they might show up

My 5th Heiny is preventing me from making any sense out of the 2nd prob.
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      05-31-2007, 03:26 AM   #8
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Quote:
Originally Posted by ThreeThirtyEye View Post
Sorry I don't have your answer but this may help:

Problem 1
force = mass x acceleration
Since speed is constant, acceleration will be gravity.
Since acceleration is gravity, the force is justified by the weight
force = 65.2 kg (9.8 m/s2)

at 10.4% more force:
just find acceleration if force is increased by 10.4%
acceleration = force(10.4%)/65.2
i tried doing 65.2*9.8 but it said that was the incorrect answer.

For part 2: whats the force that i use?
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      05-31-2007, 04:07 AM   #9
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Im out for the night, if any1 comes up with the answer please post
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      05-31-2007, 05:22 AM   #10
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Quote:
Originally Posted by BMWPower06 View Post
i tried doing 65.2*9.8 but it said that was the incorrect answer.

For part 2: whats the force that i use?
Non conservative and conservative forces
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      05-31-2007, 09:15 AM   #11
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I'm at work and the pictures are not working for some reason... In any case, for the first one, are there pulleys involved? If so, you need to include the mechanical advantage that you gain by including pulleys in the system.. I think that is the link you are missing..
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      05-31-2007, 11:35 AM   #12
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Basically she has to overcome her true weight which is 65.2kg x 9.8m/s^2
and also note that the speed is constant so there is NO acceleration in the first part of this question.
Therefore answer is:

F = ma
= 65.2 x 9.8
= 638.96N downwards. (this is the force she needs to pull the rope at)

By increasing the force by 10.4% we achieve a force of 705.41N downwards (by calculation). Then:

a = F/m
= 705.41/65.2
= 10.8 m/s^2


But you said this gave the wrong answer. Hmm, I’m not sure but I would think it is the right answer. She has to overcome her true weight (mass x gravitational force) to get a constant upwards speed. A constant upwards speed also means there is NO acceleration. Then you just have to calculate 10.4% of your answer from part 1 and then add it to the answer from part 1, this will give you the total force. Then just use a = F/m to calculate the acceleration. That is what I would do, but you said it’s wrong…
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