## Question

A 120 metre long train is moving west at a speed of 10 ms^{–1}. A small bird flying east at a speed of 5 ms^{–1} crosses the train. What is the time taken by the bird to across the train?

### Solution

8 *s*

The relative speed of the bird

.

Therefore, time taken by the bird to cross the train is

.

#### SIMILAR QUESTIONS

Car *A* is moving with a speed of 36 km h^{–1} on a two-lane road. Two cars*B* and *C*, each moving with a speed of 54 km h^{–1} in opposite direction on the other lane are approaching car *A*. At a certain instant when the distance *AB* = distance *AC* = 1 km, the diver of car *B* decides to overtake*A* before *C* does. What must be the minimum acceleration of car *B* so as to avoid an accident?

The driver of a train *A* moving at a speed of 30 ms^{–1} sights another train*B* moving on the same track at a speed of 10 ms^{–1} in the same direction. He immediately applies brakes and achieves a uniform retardation of 2 ms^{–1}. To avoid collision, what must be the minimum distance between the trains?

The driver of a train *A* moving at a speed of 30 ms^{–1} sights another train*B* moving on the same tack towards his train at a speed of 10 ms^{–1}. He immediately applies brakes and achieves a uniform retardation of 4 ms^{–2}. To avoid head-on collision, what must be the minimum distance between the trains?

Rain is falling vertically with a speed of 4 ms^{–1}. After some time, wind starts blowing with a speed of 3 ms^{–1} in the north to south direction. In order to protect himself from rain, a man standing on the ground should hold his umbrella at an angle given by

A swimmer can swim in still water with a speed of 5 ms^{–1}. While crossing a river his average speed is 3 ms^{–1}. If he crosses the river in the shortest possible time, what is the speed of flow of water?