Question

Jackie Chan is filming a stunt for his new movie The Last Emperor of Des Moines. In the scene, Jackie is running across the top of a charter bus as it speeds down the highway directly into the path of an oncoming semi-truck. The bus is traveling at 20 m/s due west, Jackie is running 2 m/s due east, and the semi-truck is traveling 15 m/s due east. Describe Jackie's motion from the perspective of the driver of the semi-truck. Be sure to explain your reasoning.

Answer 1

33m/s to the west

Step-by-step explanation:

To describe the motion of Jackie you use the simple sum of velocities in a classical frame of reference. First of all you compute the speed of Jackie measured by an observer at rest. This velocity is the difference between the velocity of the bus and the velocity of Jackie because their motion are opposite.

`v_J'=v_B-v_J`

v'J: velocity of Jackie as seen by an observer at rest

vJ: velocity of Jackie = 2m/s

vB: Bus velocity = 20m/s

By replacing you obtain:

`v_J'=20m/s-2m/s` = 18m/s

Now, due to the semi-truck is traveling in the same direction of Jackie, you sum this velocities, in order to calculate the velocity of Jackie as seen by the driver in the semi-truck:

`v_J''=v_J'+v_s=18m/s+15m/s=33m/s`

hence, the driver of semi-truck sees that Jackie is moving with a velocity of 33m/s to the west