>>11824802Think of it like this.
1. Person A is stationary on the side of the road. He throws a ball from his waist to the height of the top of his head. Measure the distance and time it takes to come back to his waist. Think of the ratio for distance traveled to time it took.
2. Person B is in a car traveling 50 mph or 50km/hr (who cares). As he's in motion, person B throws a ball from his waist to the height of the top of his head. Exact same as Person A. Measure the distance of how far the ball traveled, and the time it took to go back to his waist.
Now, question. From the perspective of Person A (who is stationary on the side of the road), what would the trajectory of Person B's ball look like? It would look like a triangle because as the ball travels from waist to head, it does so while the car is traveling forward. Think of the ratio for distance traveled to time it took to throw and catch it.
Now if that's the case, that means that in order for Person A and Person B's ratio for distance traveled compared to time it took to throw and catch the ball, to match each other's it must mean that the Person traveling (person B) must have had an equivalent extension of time just like Person B had an extension of distance the ball traveled.
Here is a video to explain how you can arrive at this same conclusion.
https://www.youtube.com/watch?v=p2nwdS3ia24