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Tension practice problem

by bulbasaur88
Tags: newton's second law, tension
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bulbasaur88
#1
Aug5-11, 07:34 PM
P: 57
I'm trying to improve on tension problems so I am practicing using this website: http://www.education.com/study-help/article/tension/

I am on problem 6, but I keep coming up with the wrong answers. I attempted this by trying to show the net forces for each box.

Box M net forces
T upwards
Mg downwards
F = T - Mg = Ma

Box 2M net forces
T upwards
T' downwards
***should I include the weight (2Mg) of this box???

Box 4M net forces
4Mg downwards
T' upwards
F = 4Mg - T' = 4Ma

I have a feeling there is an error in box 2M that is fudging with my results. Lastly, does T = 6Mg? Help??
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PhanthomJay
#2
Aug5-11, 07:46 PM
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Quote Quote by bulbasaur88 View Post
I'm trying to improve on tension problems so I am practicing using this website: http://www.education.com/study-help/article/tension/

I am on problem 6, but I keep coming up with the wrong answers. I attempted this by trying to show the net forces for each box.

Box M net forces
T upwards
Mg downwards
F = T - Mg = Ma
yes

Box 2M net forces
T upwards
T' downwards
***should I include the weight (2Mg) of this box???
always include the weight in any free body diagram

Box 4M net forces
4Mg downwards
T' upwards
F = 4Mg - T' = 4Ma
yes

I have a feeling there is an error in box 2M that is fudging with my results. Lastly, does T = 6Mg? Help??
yes, correct your error for the net forces acting in the FBD of the 2M block.
bulbasaur88
#3
Aug5-11, 07:52 PM
P: 57
was my error that I didn't include the weight? or is it another entirely separate error in block 2m?

bulbasaur88
#4
Aug5-11, 07:54 PM
P: 57
Tension practice problem

Is the equation for block 2m:

F = 2Mg + T' - T = 2Ma?
PhanthomJay
#5
Aug5-11, 08:12 PM
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Quote Quote by bulbasaur88 View Post
Is the equation for block 2m:

F = 2Mg + T' - T = 2Ma?
yes. Now solve for T, T', and a. (Note: T is not equal to 6Mg)
bulbasaur88
#6
Aug5-11, 08:27 PM
P: 57
I solved it and it still incorrect.

Block M
T - mg = ma <---plug in m = 1 Kg
T - 9.8 = a
T = 9.8 + a equation 1

Block 2M
T' + 2mg - T = 2ma <---Plug in m = 1 kg
T' + 2(9.8) - T = 2a
T' + 19.6 - T = 2a equation 2

Block 4M
4mg - T' = 4ma < --- plug in m = 1 kg
4(9.8) - T' = 4a
T' = 39.2 - 4a equation 3

Substitute equation 1 and equation 3 into equation 2
equation 2: T' + 19.6 - T = 2a
(39.2 - 4a) + 19.6 - (9.8 + a) = 2a
Solve for a.

a = 7 m/s^2

Correction answer is 8.6 m/s^2.
bulbasaur88
#7
Aug5-11, 08:43 PM
P: 57
We were correct :D

The answer provided is incorrect I believe. Someone solved it out and got the same answer as I did here, http://www.myteacherpages.com/webpag...0SOLUTIONS.pdf.
bulbasaur88
#8
Aug5-11, 08:44 PM
P: 57
Thank you phanthomjay *air hug*


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