Question

Alex, parked by the side of an east-west road, is watching car P, which is moving in a westerly direction. Barbara, driving east at a speed 52 km/h, watches the same car. Take the easterly direction as positive. If Alex measures a speed of 78 km/h for car P, what velocity will Barbara measure?

Answer 1

`v_(PB) = 130\ km/h`

Step-by-step explanation:

Since, Alex is at rest. Therefore, the speed measured by him will be the absolute speed of car P. Therefore, taking easterly direction as positive:

`Absolute\ Velocity\ of\ Car\ P = v_(P) = -78\ km/h`

And the absolute velocity of Barbara's Car is given as:
`Absolute\ Velocity\ of\ Barbara's\ Car = v_(B) = 52\ km/h`

Now, for the velocity of Car p with respect to the velocity of Barbara's Car can be given s follows:

`Velocity\ of\ Car\ P\ measured\ by\ Barbara = v_(PB) = v_(B)-v_(P)\\\\v_(PB) = 52\ km/h-(-78\ km/h)`

`v_(PB) = 130\ km/h`