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Name: Brandon
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Question:
I have a question that me and my wife have been debating on for years now. My questions is if a train is going say 50 mph. and a baseball is thrown say 90 mph. in the direction the train is traveling how fast is the ball really traveling? I do believe inside the train the ball will be going 90 mph, but if there was a way to get an accurate reading on the ball from outside the train would it be going 140 mph. Someone please help us with this physics question.


Replies:
You need to select a frame of reference. This is very important.

Relative to the train, the ball is going 90. Relative to the ground, it can be 90 + 50 (forward); 90 - 50 (backward). This simple addition (often referred to as Galilean Relativity) breaks down at high speeds (close to the speed of light).

---Nathan A. Unterman



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