Determine the average spacing between cars of a row
proceeding at 60 km/h, if the driver of a car travelling
at 80 km/h in the opposite direction passes by 14 cars
of the row per kilometre. (the average length of the
cars is 5 m.)

Explanation:

Let us assume the average spacing between cars to be .

Frame of Reference : Frame attached to Cars moving at .

   In this frame, the cars in the row are at rest, while the car travelling in opposite direction appears to move at a relative speed of .

   He is covering a distance of in ground frame. His speed in ground frame is . So, time taken to traverse 1 km is

   In this T time, he moves in the relative frame at . The distance covered in relative frame will be

×

×

   It passes 14 cars while traversing this distance.

∴ 14 ×

×

.

∴Average Spacing betwwen cars in a row =

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it

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