A stone is moved in a horizontal circle of radius 4 m by means of a string at a height of 20 m above the ground. The string breaks and the particle files off horizontally, striking the ground 10 m away. The centripetal acceleration during circular motion is

1. 6.25 

2.12.5 

3. 18.75 

4. 25 

 

Three long wires, each carrying current i are placed parallel to each other. The distance between I and II is 3d, between II and III is 4 d and between III and I is 5d. Magnetic field at side of wire II is 

1. $\frac{5{\mathrm{\mu }}_0\mathrm{i}}{24\mathrm{\pi d}}$

2. $\frac{10{\mathrm{\mu }}_0\mathrm{i}}{24\mathrm{\pi d}}$

3. $\frac{15{\mathrm{\mu }}_0\mathrm{i}}{24\mathrm{\pi d}}$

4. $\frac{20{\mathrm{\mu }}_0\mathrm{i}}{24\mathrm{\pi d}}$

In the following diagrams, a particle with small charge – q is free to move up or down, but not sideways near a larger fixed charge Q. The small charge is in equilibrium because in the positions shown, the electrical upward force is equal to the weight of the particle. Which statement is true? 

1. In figure a, -q is in stable equilibrium.

2. In figure a, -q is in neutral equilibrium.

3. In figure b, -q is in stable equilibrium.

4. Neither in fig(a) nor in fig(b) , q is in stable equilibrium.