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Sometimes in physics questions we see examples in which applied force is balanced by an innate force of the body. Like in torsion balances and Cavendish's experiment. In this case we say that rotation( of coil) was up to the point where our applied force balanced the innate restoring force & there is no oscillation in this case. But in certain cases we see that instead of rotation & then stopping the coil rotates. For eg: in a ballistic galvanometer.

So why in similar cases there is rotation in one case & oscillation in other cases ? What causes oscillation in not just the case of a B.G but other things also ( please give other eg also) & what causes just rotation & no oscillation in other one (please give other eg also) ?

Consider the example of an ammeter , the coil rotates till the external torque ( due to current) equals the torsion couple. But in a B.G the coil oscillates though charge ( or current has passed). It doesn't stop the cases are similar but results are different.

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For the motion to be oscillatory two conditions should be met. 1. There should be a return force that returns the system back to equilibrium. 2. The system should have an inertia, that is once it is in equilibrium position with a certain velocity it should continue to move. Inertia takes the form of inductance in a LC electric circuit and capacitor the role of return force. Damping takes away the energy from the oscillating system. With too much damping oscillations are barely possible. Similar connection can be made for other systems.

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  • $\begingroup$ How should I use this to explain the above eg of a torsion balance (no rotation ) & B.G( oscillation) $\endgroup$ – E2n Jan 24 '17 at 14:36
  • $\begingroup$ What you are doing in these Cavendish experiments is that you have moved the system away from equilibrium and has not released it but got it fixed. $\endgroup$ – SAKhan Jan 24 '17 at 17:33
  • $\begingroup$ No we never fix the torsion. It rotates and then stops. Consider the example of an ammeter , the coil rotates till the external torque ( due to current) equals the torsion couple. But in a B.G the coil oscillates though charge ( or current has passed). It doesn't stop the cases are similar but results are different. $\endgroup$ – E2n Jan 25 '17 at 15:20
  • $\begingroup$ When I say you fix it I mean you apply a force (gravitation or electrical) to displace it from equilibrium position and don't remove that force. If the force is removed it would oscillate. $\endgroup$ – SAKhan Jan 25 '17 at 16:20

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