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If I'm using a servo with a string attached to it (while the string is attached to another object) the string is attached tangentially at about 1.5cm from the centre of the servo and the servo draws 270 milli Amps and has 5V supplied to it (while having an angular velocity of pi/10) can I find the force in the string by doing the following: using P=IV find the power of the servo then using angular velocity = linear velocity x radius work out the linear velocity of the string, then using P = FV find the force in the string?

Thanks for helping out!

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Short answer is no.

Although $P= IV$ is related to the energy and theoretically you can find the Force through the use of $P= F\cdot V$ there are at least two problems both related with the conversion of electrical energy to actual work:

  1. Friction and losses in general.

Some The electrical energy is converted into thermal energy through friction. Although most of the times this is negligible, in truth there is no easy way to quantify that.

  1. power factor or Cos $\phi$

You probably know that the power factor is defined as $P=\frac{\text{active power }[W]}{\text{apparent power}[VA]}$. The cos $\phi$ indicates how much apparent power is converted to actual power. The remaining power is usually stored as inductive or capacitative energy in the circuit.

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  • $\begingroup$ Ah ok cool, but how can I find Phi? Also so if I mulitiply my current through and volatge across my servo is that my apparent power? Also if Im just expalining my idea (to a physicist say) and ignore real life aspects (such as energy loss) will my method work? $\endgroup$
    – Vortex
    Mar 2, 2021 at 14:52
  • $\begingroup$ in larger motor you invariably find it in the label of the motor (see updated image) in answer. However, keep in mind that this is an average measurement. It can actually fluctuate within a second. $\endgroup$
    – NMech ♦
    Mar 2, 2021 at 16:51
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I am questioning the wisdom of using the power equation, $P = FV$, for this case, as the known velocity is "angular velocity, $\omega$, but the linear velocity, $v$.

Instead, I think you shall use the equation $P = \tau \omega$ to find the force in the string. Here $\tau$ is the torque about the center of the servo. Please correct me if I am wrong.

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