Net Forces and Tension Practice Problems

[bulbasaur88]

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??

 

[PhanthomJay]

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]

was my error that I didn't include the weight? or is it another entirely separate error in block 2m?

 

[bulbasaur88]

Is the equation for block 2m:

F = 2Mg + T' - T = 2Ma?

 

[PhanthomJay]

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]

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]

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/webpages/niedzielam/files/AP%20Review%2001%20Tension%20SOLUTIONS.pdf .

 

[bulbasaur88]

Thank you phanthomjay *air hug*