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How do I calculate the tension of the string?

How do I calculate the tension of string? I have already calculated the acceleration of the 2 masses which is 3m/s. But I can't seem to arrive at the answer which is 9.00N. Sorry if this was a straightforward question.

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    $\begingroup$ Do a free body diagram on the 3kg block - what force would be required to accelerate a 3kg block at the acceleration you calculated? $\endgroup$
    – Señor O
    May 2 at 3:48
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    $\begingroup$ It would be 9N, is that the tension? $\endgroup$
    – Ja_____en
    May 2 at 3:51

1 Answer 1

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As you correctly found acceleration of the centre of mass, and hence of both the blocks (as both must be moving with same acceleration to keep the string taught) is $3m/s^2$. Now draw a free body on 3kg mass, it is moving with $3m/s^2$ acceleration with only force acting on it i.e tension. So tension = 3*3=9N

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  • $\begingroup$ Oh, understood. Thanks! $\endgroup$
    – Ja_____en
    May 2 at 4:07
  • $\begingroup$ Wait, i understand why the 3kg mass has only one force on it which is the tension, but what about the 5kg mass on the right? Why does it not be included in the tension? $\endgroup$
    – Ja_____en
    May 2 at 4:10
  • $\begingroup$ @Ja_____en It is included in the tension, whats its acceleration 3 as well so net force on it must be 15. and 24-T is the net force on it = 15, hence T=9. You can solve it using either mass both are consistent $\endgroup$ May 3 at 9:10

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