As I understand it If there was a train traveling at 100 miles per hour you fire a gun off the back of that train the bullet will travel 100 miles per hour if you where not on the train it would appear to just stop and fall to the ground if it was fired from the front the bullet would be traveling at 200 miles per hour.

If you are in a car (hard top) and you through a ball in the air it will come back down instead of flying to the back of the car because the ball is traveling at the same speed as the car.

Sorry I am not that good at metaphors.

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