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During a discussion with a friend on the equivalence principle, the point was made that it only applies to objects in free fall, and therefore wouldn't apply to objects on an inclined plane. In other words, there would be a difference between how an object would fall on an inclined plane in a gravitational field and how it would it fall in an accelerating frame without gravity.

Is this correct? And if so, how could one determine what that difference would be?

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No, there is no observable difference and you've answered it yourself - it's due to equivalence principle.

If, however, observer is influenced by gravity and accelerated simultaneously and both forces are in an angle, then the direction and magnitude of the resulting acceleration can be calculated by vector addition.

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  • $\begingroup$ Thanks for the response. I'm having trouble wrapping my head around how anything would fall without any net downward force on the object. I get it when an object is in free fall, but my understanding is that the equivalence principle wouldn't apply here because an object on an incline isn't in free fall. What am I missing? $\endgroup$
    – Seemom
    Commented Nov 9, 2020 at 15:50
  • $\begingroup$ Free fall is just a downwards acceleration. Any other direction of acceleration is also an acceleration with the same consequences. Any acceleration needs a force, even free fall (due to Newtons first law of motion). $\endgroup$
    – Krumuvecis
    Commented Nov 9, 2020 at 15:58

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