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Math Help - Calculating Tension

  1. #1
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    Calculating Tension

    Hi, I'm trying to do this physics problem, but I just can't seem to figure out how to do this problem involving tension. I think it has something to do with adding the masses together on either side of Tc, but I can't seem to come up with the correct answer.
    Any help is greatly appreciated, thanks!
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  2. #2
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    Quote Originally Posted by vesperka View Post
    Hi, I'm trying to do this physics problem, but I just can't seem to figure out how to do this problem involving tension. I think it has something to do with adding the masses together on either side of Tc, but I can't seem to come up with the correct answer.
    Any help is greatly appreciated, thanks!
    Assuming that the strings are inextensible (and I guess they will be if they're massless ha ha), the simplest approach is:

    Start off by considering the four masses as a single system under a net force of 27 newton to the left. Then the acceleration of the system is 3 m/s^2 to the left.

    Now consider the net force on the 5 kg mass. Keep in mind that it has an acceleration of 3 m/s^2 to the left .....
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    So using F=ma, I could plug in a=3 and m=5 to get Tc=15N?
    Or would I plug in a mass of 4kg to get Tc=12N?
    Thanks for the help, I think I'm starting to understand it now.
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  4. #4
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    Quote Originally Posted by vesperka View Post
    So using F=ma, I could plug in a=3 and m=5 to get Tc=15N?
    Or would I plug in a mass of 4kg to get Tc=12N?
    [snip]
    No and no!!

    Take left as the positive direction. Then the net force on the 5 kg mass is T_c - 14 newton.

    But net force = (mass)(acceleration) = (5)(3) = 15 newton.

    Therefore .....
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    So I would add 15N to the 14N to get 29N? Is that correct?
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    Quote Originally Posted by vesperka View Post
    So I would add 15N to the 14N to get 29N? Is that correct?
    Hmmm lessee now ..... T_c - 14 = 15 \therefore T_c = ...... by Jove, you're right!!
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  7. #7
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    Thanks for the help!
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