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 force on the thing and the distance the force moves the thing