A person riding a merry-go-round passes very close to a person standing on the ground once (event $A$ ) and then again (event $B$ ). Assume the ground is an inertial frame and that the rider moves at a constant speed.
-(b) Which person's watch measures the spacetime interval $\Delta s$ between those events?
A) The rider in the merry-go-round
B)The person standing on the ground
C) Both
D) Neither

Question

Quizplus · Accepted Answer

The spacetime interval $\Delta s$ between two events is invariant for all observers, but in the context of whose clock measures the proper time (the time between two events occurring at the same point in space for that observer), it is the person standing on the ground. This is because for the person on the ground, events A and B occur at the same spatial location (where they are standing), making their frame an inertial frame where the proper time is measured. The rider on the merry-go-round is moving, so the events occur at different spatial locations for them.

A Person Riding a Merry-Go-Round Passes Very Close to a Person