21.In an atom two electrons move round the nucleus in circular orbits of radii R and 4R respectively. The ratio of the time taken by them to complete one revolution is:

22.Mark correct option or options from the following.

23.A stone of mass 1 kg is tied to the end of a string 1 m long. It is whirled in a vertical circle. The velocity of the stone at the bottom of the circle is just sufficient to take it to the top of circle without slackening of the string. What is the tension in the string at the top of the circle? (Take g = 10 m/s)

24.A particle is acted upon by a force of constant magnitude which is always perpendicular to the velocity of the particle. The motion of the particle takes place in a plane. It follows that:

25.The maximum speed with which an automobile can round a curve of radius 8 m without slipping if the road is unbanked and the co-efficient of friction between the road and the tyres is 0.8, is: (g = 10 m/second square)