The vibrations of a string of length 60 cm fixed at both ends are represented by the equation
$$y = 4\sin \left( {{{\pi x} \over {15}}} \right)\cos (96\pi t)$$
where x and y are in cm and t is in second. Calculate the velocity of the particle at x = 7.5 cm at t = 0.25 s.
A charge +q is placed at the origin O of XY-axes as shown in the figure. The work done in taking a charge Q from A to B along the straight line AB is
Identify the hydrogen-like element whose spectral lines are four times shorter in wavelength compared to those of atomic hydrogen.
Two small conducting spheres of equal radius have charges +20 $$\mu$$ C and $$-$$40 $$\mu$$C respectively and placed at a distance R from each other experience for F1. If they are brought in contact and separated to the same distance, they experience force F2. The ratio of F1 to F2 is