Simple Machines B/Compound Machines C

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Re: Simple Machines B/Compound Machines C

Post by mjcox2000 » April 4th, 2015, 6:47 am

I was intending for one of the two of you to post a question, but I can do another one if you'd like - what would you prefer?
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Re: Simple Machines B/Compound Machines C

Post by UTF-8 U+6211 U+662F » April 4th, 2015, 3:01 pm

mjcox2000 wrote:I was intending for one of the two of you to post a question, but I can do another one if you'd like - what would you prefer?
I'm a little bit, err.... preoccupied (code phrase for "I'm on vacation"). Why don't you post a question?

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Re: Simple Machines B/Compound Machines C

Post by Unome » April 4th, 2015, 3:02 pm

Ok then:

If M2 has a mass of 20.0 kg, and for all pulleys the mass is 1.00 kg and the efficiency is 90.0%, solve for the mass of M1 needed to balance the system (with proper significant figures) (assume all ropes are exactly vertical; I'm not sure how them being at angles would affect the system).
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Re: Simple Machines B/Compound Machines C

Post by UTF-8 U+6211 U+662F » April 4th, 2015, 3:59 pm

Unome wrote:Ok then:

If M2 has a mass of 20.0 kg, and for all pulleys the mass is 1.00 kg and the efficiency is 90.0%, solve for the mass of M1 needed to balance the system (with proper significant figures) (assume all ropes are exactly vertical; I'm not sure how them being at angles would affect the system).
So, I'm guessing I have to use gravity:
IMA = 2*1*2*1*1 = 4 20.0/4.00=5.00 5.00*0.90=[b]4.5 kg[/b]
I don't remember how to calculate with gravity. Oh well.

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Re: Simple Machines B/Compound Machines C

Post by Unome » April 4th, 2015, 4:22 pm

UTF-8 U+6211 U+662F wrote:
Unome wrote:Ok then:

If M2 has a mass of 20.0 kg, and for all pulleys the mass is 1.00 kg and the efficiency is 90.0%, solve for the mass of M1 needed to balance the system (with proper significant figures) (assume all ropes are exactly vertical; I'm not sure how them being at angles would affect the system).
So, I'm guessing I have to use gravity:
IMA = 2*1*2*1*1 = 4 20.0/4.00=5.00 5.00*0.90=[b]4.5 kg[/b]
I don't remember how to calculate with gravity. Oh well.
Incorrect; You don't need gravity to do this.
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Re: Simple Machines B/Compound Machines C

Post by UTF-8 U+6211 U+662F » April 4th, 2015, 4:44 pm

Unome wrote:
UTF-8 U+6211 U+662F wrote:
Unome wrote:Ok then:

If M2 has a mass of 20.0 kg, and for all pulleys the mass is 1.00 kg and the efficiency is 90.0%, solve for the mass of M1 needed to balance the system (with proper significant figures) (assume all ropes are exactly vertical; I'm not sure how them being at angles would affect the system).
So, I'm guessing I have to use gravity:
IMA = 2*1*2*1*1 = 4 20.0/4.00=5.00 5.00*0.90=[b]4.5 kg[/b]
I don't remember how to calculate with gravity. Oh well.
Incorrect; You don't need gravity to do this.
20.0+1.00=21 21/2=10.5 10.5*2=21.0 21.0/0.90=[b]23.3 kg[/b]
Probably not right... I can't figure out the MA for the second pulley from the rigt.

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Re: Simple Machines B/Compound Machines C

Post by Unome » April 5th, 2015, 9:45 am

UTF-8 U+6211 U+662F wrote:
Unome wrote:
UTF-8 U+6211 U+662F wrote: So, I'm guessing I have to use gravity:
IMA = 2*1*2*1*1 = 4 20.0/4.00=5.00 5.00*0.90=[b]4.5 kg[/b]
I don't remember how to calculate with gravity. Oh well.
Incorrect; You don't need gravity to do this.
20.0+1.00=21 21/2=10.5 10.5*2=21.0 21.0/0.90=[b]23.3 kg[/b]
Probably not right... I can't figure out the MA for the second pulley from the right.
Nope. Try doing it backwards or upside down; it helps to have a comparison. Also, every pulley has a separate efficiency of 90%, so they have to be calculated one by one. Thinking about how efficiency actually functions would also help.
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Re: Simple Machines B/Compound Machines C

Post by UTF-8 U+6211 U+662F » April 5th, 2015, 2:25 pm

20/2=10 10/0.9=100/9 (100/9)/0.9=1000/81 (1000/81)/0.9=10000/729 10000/729*2=20000/729 (20000/729)/0.9=200000/6561=[b]30.5 kg[/b]

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Re: Simple Machines B/Compound Machines C

Post by Unome » April 6th, 2015, 12:11 pm

Unless I'm doing it wrong (and I don't think I am) no. I think you're messing up somewhere in the beginning.
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Re: Simple Machines B/Compound Machines C

Post by UTF-8 U+6211 U+662F » April 6th, 2015, 12:29 pm

Unome wrote:Unless I'm doing it wrong (and I don't think I am) no. I think you're messing up somewhere in the beginning.
Are the masses of the pulleys relevant?
7.62 kg? I'm honestly thinking of just giving up and posting another question.

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