Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction.

The radiation emitted by a star $A$ is $10,000$ times that of the sun. If the surface temperatures of the sun and the star $A$ are $6000 \ K$ and $2000 \ K$ respectively,the ratio of the radii of the star $A$ and the sun is (in $:1$)

Temperature of cold junction in a thermocouple is $10^{\circ} C$ and neutral temperature is $270^{\circ} C$,then the temperature of inversion is (in $^{\circ} C$)

One end of a string of length $l$ is connected to a particle of mass $m$ and the other to a small peg on a smooth horizontal table. If the particle moves in a circle with speed $v$ and $T$ is the tension in the string,then the net force on the particle (directed towards the centre) is

The rate of steady volume flow of water through a capillary tube of length $l$ and radius $r$, under a pressure difference of $p$ is $V$. This tube is connected with another tube of the same length but half the radius, in series. Then, the rate of steady volume flow through them is (The pressure difference across the combination is $p$.)

$ATP$ase enzyme needed for muscle contraction is located in