**GeistKeisel:**The cyclist and the block of lead problem is easily explained via application of General Relativity, which means that ordinary people like you and I cannot understand the explanation. I will give you a general (pun intended) idea, and ignore the issue of light reflected back and forth in a room. Perhaps I might address the latter issue in another post.

Your description of the cyclist and the block of lead refers to acceleration. Gravity and acceleration are dealt with by General Relativity. Changing from the usual frame of reference to one in which the cyclist is stationary results in attributing the velocity and acceleration of the cyclist to the block of lead as well as to everything else in the universe.

When General relativity is applied to this problem, the equations indicate the following.

- In one frame of reference, the cyclist feels the inertial effects of acceleration due to his motion.

- In the other frame of reference, the cyclist feels gravitational effects due to being kept stationary while the universe is moving.

- In both frames of reference, the block of lead feels gravitational effects due to being stationary in the Earth’s gravitational field.

- When the cyclist is viewed as stationary, the block of lead is viewed as falling in the gravitational field due to the cyclist. In this frame of reference, the block of lead feels no forces or effects due to the cyclist.

- When the cyclist is viewed as moving, I am not sure how to describe his effects on the block of lead, but they should be null for consistency with the other point of view.

I do feel very comfortable with the concept of the equivalence of inertial (accelerative) and gravitational effects. Once described, they seem intuitively correct. If in a room sitting on the surface of the Earth, I am aware of gravitational effects. If in a similar room aboard an accelerating rocket ship in empty space, I am aware of inertial effects. From inside either room, I cannot decide which room I am in. Also while falling in a gravitational field, all the effects seem equivalent to being motionless in empty space: In either condition, I sense no forces acting on me.

BTW: An old bit of folk wisdom is more significant than it appears to be.

**It is not the fall that hurts. It is the sudden stop that does all the damage.**

Thinking about the above initiated the development of GR, and those equivalences are the basis for explaining the situation relating to the cyclist and the block of lead.

Note that mass appears in two important equations: The classical force equals mass times acceleration equation and the Newtonian equation for force due to a gravitational field. There is no reason to assume that both equations relate to the same attribute of matter.

Deep thinkers viewed the situation as a cosmic coincidence, while ordinary students merely learned the equations without thinking much at all. Einstein pondered about the equations and realized that the equivalence of inertial and gravitational mass was a clue to very important but yet unknown concepts. He next thought about the observers inside rooms either being accelerated or at rest in a gravitational field. Many years later, he published his thoughts on GR.