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

Answer:

$v_{PB} = 130\ km/h$

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:

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

$v_{PB} = 130\ km/h$