The object or objects in motion here will depend on which of them you are relating motion to. If standing on the ball, the two bars will be the ones moving at 50mph at one direction, if standing on ether of the bars, the ball will be the one moving at 50mph at the opposite direction of the bars. This factor is why if you were moving at a fixed speed in a closed car, through a very steady, straight road, no matter how high the speed of the car might be, if a ball were to be thrown up to the air, it would always fall as normally as if you were standing on still ground, for, in every sense of the way, you would be, except, of coarse, for any shakes which the car may have because of the road. So as long as the speed of a mass is fixed, the mass is standing still relative to itself. But what if a mass were to increase its speed.
Lets take, for example, a car that is about to travel to a park.
As the car begins to accelerate, the driver begins to feel a little squeezed because of the force known as inertia, which would be traveling at the opposite direction of the car's accelerating motion. While the car accelerates in a straight direction, its relativity to the space being covered in time begins to shrink, for it is taking the car less time to reach its destination as its speed increases. Therefore, the car will no longer be standing still relative to itself, for it will now be moving through different relative time space points of motion whereby the present space in time is always smaller relative to the past one. So, when a mass is going through the acceleration process, it keeps moving to a different relative time space point where motion in time is faster, thereby, making relative time space shrink at the direction of motion which the mass travels at, creating during the proceedings of this event the force of inertia. Now, lets imagine that the car has reached its fixed level of 50mph.