# 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° angle to the horizontal when the car is moving faster than 30 km per hour. Find the velocity of rain droplets.

Expert's answer

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°)=|V|/|U|.

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

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

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°)=|V|/|U|.

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

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

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