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

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

Step-by-step 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.