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.
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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Update: June 2012