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Using your relativity formulas, figure out how far the explorer has gone after one week. Then divide that distance by 186,000 and you'll get how many seconds it takes the message to reach the planet. The reverse trip... I'm not so sure on. If I weren't so focused on other areas of physics atm...

When viewed from certain angles, the spot would not be visible due to bending (refraction) of light; however, the spot would be visible at certain other points. It's asking what must be 'blacked out,' so to speak, in order for the spot never to be seen from the areas where refraction wouldn't...

Using formula: d=(v_i*t)+(.5at^2) d_greentrain = 800m ((40*40)+(.5(-1)(40)^2))=800) d_redtrain = 200m ((20*20)+(.5(-1)(20)^2))=200) Since d_g+d_r = 1000 and 1000m > 950m, the trains collide. As to where... since the red train stops after only 200m, it is a safe assumption that...

Finding the time it takes to stop doesn't, but considering that their closing velocity is 60m/s, and that they're slowing at a combined rate of 2m/s^2... If they didn't slow down at all, they would collide in 15.83 seonds. Bah, I can't remember my sum formulas from HS Calc either... I'll have to...

So, I searched and found one almost identical to this, but no direction towards solution was posted, and I couldn't make sense of the formula in the original post. So... A red train traveling at 72km/h and a green train traveling at 144km/h are headed toward one another along a straight level...