Three motor vehicles travel along a straight desert highway at the constant speeds listed below (all stated relative to Earth):
• Sports car S: 25 m/s toward the east
• Delivery truck D: 15 m/s toward the east
• Motorcycle M: 30 m/s toward the west
For the duration of the scenario, each driver continually observes the delivery truck.
(a) Rank, from greatest to least, the magnitude of the velocity of the delivery truck as measured by each of the following observers:
(1) The driver of the sports car S
(2) The driver of the motorcycle M
(3) An observer standing on the ground beside the highway
State any ties explicitly.
(b) State which, if any, observer listed in part (a) sees the delivery truck momentarily at rest. Explain your answer without performing detailed numerical calculations.
(c) Suppose that, at a later time, all three vehicles instantaneously double their speeds while keeping the same directions of travel. Without plugging in numbers, describe how your answers to parts (a) and (b) would change, if at all. Provide a clear justification based on reference-frame reasoning.