Answer:

According to Einstein’s theory of special relativity, the length of objects moving at relativistic speeds undergoes a contraction along the dimension of motion. An observer at rest (relative to the moving object) would observe the moving object to be shorter in length1. The formula for length contraction is given by:

L = L0 * sqrt(1 - v2/c2)

where L is the length of the rod as measured by an observer in motion relative to it, L0 is the length of the rod as measured by an observer at rest relative to it, v is the velocity of the rod relative to the observer in motion, and c is the speed of light1.

Given that the rod has a length of 5m and is moving with a speed v=0.8c, we can calculate its length contraction as follows:

L = 5 * sqrt(1 - (0.8c)2/c2) = 3 m

Therefore, the percentage of length constriction is:

(5-3)/5 * 100% = 40%

So, the length of the rod will appear to be 3 meters long when observed by an observer in motion relative to it1.

I hope this helps! Let me know if you have any other questions.