## Question

A train is moving southwards at a speed of 30 ms^{–1}. A monkey is running northwards on the roof of the train with a speed of 5ms^{–1}. What is the velocity of the monkey as obserced by a person standing on the ground?

### Solution

25 ms^{–1} in the southward direction

Suppose we choose the direction from south to north as the positive direction. Then the velocity of the train moving southwards = –30 ms^{–1}. Velocity of the monkey running northwards = +5 ms^{–1}. Therefore, the velocity of the monkey as observed by a person in the ground = –30 + 5 = –25 ms^{–1}. The negative sign indicates that the direction of this velocity is southwards.

#### SIMILAR QUESTIONS

A man wants to reach point B on the opposite bank of a river flowing at a speed. What minimum speed relative to water should the man have so that he can reach point B. In which direction should he swim?

A train is moving eastwards with a velocity of 10 m/sec. On a parallel track another train passes with a velocity 15 m/sec eastwards. To the passengers in the second train the first train appears to be moving with a velocity.

A man can swim at a speed of 3 km/hr in still water. He wants to cross a 500 m wide river flowing at 2 km/hr. He keeps himself always at angle of 120o with the river flow when swimming find the time he takes to cross the river.

A jet airplane traveling from east to west at a speed of 500 kmh^{–1} ejects out gases of combustion at a speed of 1500 kmh^{–1} with respect to the jet plane. What is the velocity of the gases with respect to an observer on the ground?

A police van moving on a highway with a speed of 36 km h^{–1} fires a bullet at a thief’s car speeding away in the same direction with a speed of 108 km h^{–1}. If the muzzle speed of the bullet is 140 ms^{–1}, with what speed will the bullet hit the thief’s car?

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?