Lately I did Francis Turbine Experiment at my institute. The objective was to find the efficiency of the turbine. Power input was found using flow rate and pressure gauge. Power output was found by using two spring balances. The spring balances were connected via a belt and wrapped around the shaft. To find the torque difference in forces on spring balances is multiplied by the radius of the shaft. For better understanding you can watch this video https://www.youtube.com/watch?v=3Is-PUC4k1o (ignore the title). I understand why they are being used but did not get the principle behind their usage. How these balances can help to find the torque. Also, when I did this experiment I saw that as net force (Resultant of Left and Right Balance) increases shaft rotational speed decreases: why is this is happening?

  • $\begingroup$ Hi Obaid and welcome to physics.SE. Please try to make your question self-contained - in particular, we should not have to watch a video to know what you are talking about. $\endgroup$
    – ACuriousMind
    Oct 19, 2016 at 16:04
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    $\begingroup$ The question is related to the video. And I think who will be able to answer this question does not have to watch the full video. He will get the full idea just by watching the first 2 mins. $\endgroup$ Oct 19, 2016 at 16:15
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    $\begingroup$ In any case, we would prefer that there's some explanation of the setup within the question - what happens if YouTube removes the video in the future, the uploader deletes it, or the link becomes dead for other reasons? $\endgroup$
    – ACuriousMind
    Oct 19, 2016 at 16:21

1 Answer 1


The output power $P$ is frictional force $F$ supplied by the belt - which is equal to the difference between the readings of the spring balance - times the speed of the belt over the shaft of the turbine, which is $R\omega$ where $R$ is radius of the shaft and $\omega$ the angular velocity :
This is just an application of the formula power=force$\times$velocity or work=force$\times$distance.

If the input power and efficiency are constant, the output power $P$ of the turbine is constant. An increase in $F$ must cause a decrease in $\omega$. ($R$ is constant.)

  • $\begingroup$ So, do you mean to say that the force supplied by the shaft is equal and opposite to frictional force supplied by the belt? $\endgroup$ Oct 19, 2016 at 18:30
  • $\begingroup$ Yes, that is what I mean. $\endgroup$ Oct 19, 2016 at 19:24

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