Question

A school bus moves past a parked car at a speed of 30 km/hr. Which of the following BEST describes the observation made by a passenger on the bus? A.The car appears to be moving 15 km/hr in the same direction as the bus.
B.The car appears to be moving 30 km/hr in the same direction as the bus.
C.The car appears to be moving 30 km/hr in the opposite direction of the bus.
D.The car appears to be moving 15 km/hr in the opposite direction of the bus.

Answer 1

Answer:

C.The car appears to be moving 30 km/hr in the opposite direction of the bus.

Explanation:

There are two reference systems involved in this situation:

- Reference system S: this is the reference system where velocities are measured with respect to the ground. In this reference system, the car is parked, so its velocity is $v=0$

- Reference system S': this is the reference system moving with the bus. This reference system is moving with a velocity of $v_0 = +30 km/h$ with respect to the reference system S

Calling $v'$ the velocity of the car in the reference system S', we have:

$v=v_0 +v'$

From which

$v'=v-v_0 = 0-(+30 km/h)=-30 km/h$

and the negative sign means that a passanger in the bus observes the car moving in the opposite direction.