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Physics : Kinematics : Relative Velocity in One and Two Dimensional Motion

Solved Example Problems for Relative Velocity in One and Two Dimensional Motion

Example 2.26

Suppose two cars A and B are moving with uniform velocities with respect to ground along parallel tracks and in the same direction. Let the velocities of A and B be 35 km h-1 due east and 40 km h-1 due east respectively. What is the relative velocity of car B with respect to A?

*Solution*

The relative velocity of B with respect to A, due east

Similarly, the relative velocity of A with respect to B i.e., due west.

Example 2.27

Suppose two trains A and B are moving with uniform velocities along parallel tracks but in opposite directions. Let the velocity of train A be 40 km h-1 due east and that of train B be 40 km h-1 due west. Calculate the relative velocities of the trains

*Solution*

Relative velocity of A with respect to B, vAB = 80 km h−1 due east

Thus to a passenger in train B, the train A will appear to move east with a velocity of 80 km h−1

The relative velocity of B with respect to A, VBA = 80 km h−1 due west

To a passenger in train A, the train B will appear to move westwards with a velocity of 80 km h−1

Example 2.28

Consider two trains A and B moving along parallel tracks with the same velocity in the same direction. Let the velocity of each train be 50 km h-1 due east. Calculate the relative velocities of the trains

*Solution*

Similarly, relative velocity of A with respect to B i.e., *v*AB is also zero.

Thus each train will appear to be at rest with respect to the other.

Example 2.29

How long will a boy sitting near the window of a train travelling at 36 km h-1 see a train passing by in the opposite direction with a speed of 18 km h-1. The length of the slow-moving train is 90 m.

*Solution*

The relative velocity of the slow-moving train with respect to the boy is = (36 + 18) km h-1 = 54 km h-1 = = 54 × 5/18 ms-1 = 15 m s-1

Since the boy will watch the full length of the other train, to find the time taken to watch the full train:

Example 2.30

A swimmer’s speed in the direction of flow of a river is 12 km h-1. Against the direction of flow of the river the swimmer’s speed is 6 km h-1. Calculate the swimmer’s speed in still water and the velocity of the river flow.

*Solution*

Let *vs* and *vr* , represent the velocities of the swimmer and river respectively with respect to ground.

When the river flow and swimmer move in the same direction, the net velocity of swimmer is 12 km p h-1.

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11th Physics : Kinematics : Solved Example Problems for Relative Velocity in One and Two Dimensional Motion |

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