**P. 4617.** A student went by bicycle to Saddle City from Frame Village. He planned to go with an average speed of 18 km/h. Because of the heavy traffic, in the first third of his journey his average speed was only 15 km/h.

*a*) What should his average speed be in the remaining part of his journey, if he is to arrive at the planned time?

*b*) What should his average speed be in the remaining time, if his speed was 15 km/h in the first one-third of the time of his journey?

(4 points)

**P. 4623.** A magnetized pin, axed through its centre of mass, swings 24 times in one minute if its axle is vertical. How many swings does the pin make if its axle is horizontal and

*a*) parallel to the magnetic meridian;

*b*) perpendicular to the magnetic meridian?

The declination of the magnetic field at the position of the observation is 60^{o}.

(5 points)

**P. 4626.** There are two alike initially uncharged spheres of radius *r* at a distance of *d**r*, as shown in the *figure.*

*a*) Switch is closed. What is the force exerted between the two spheres?

*b*) What would be the force between the two spheres, if in the previous experiment instead of closing switch , switch was closed?

*c*) Determine the force exerted between the spheres if both switches are closed.

(6 points)