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# Answer to Question #3863 in Mechanics | Relativity for mia

Question #3863
The driver notices that the rain leaves no trace on the back windshield of his car slanted at a 60&deg; angle to the horizontal when the car is moving faster than 30 km per hour. Find the velocity of rain droplets.
1
2011-08-25T13:36:41-0400
Let V be the vector of velocity of rain droplets directed downwards, U - vector of
velocity of the car.
If we consider the car as a moving system of coordinates, then the velocity of the rain will be the sum of 2 perpendicular vectors: U+V.
As the droplets leave no trace on the back windshield of the car, the resulting vector is parallel to the windshield.
So tan(60&deg;﻿)=|V|/|U|.

Then |V|=tan(60&deg;﻿)*|U| = sqrt(3)*30 = 52 km/h.

So, the velocity of rain droplets is 52 km per hour.﻿

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