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#### AStudentNeedsHelp

Hi, I have recently been learning relative acceleration and I have a bit of trouble with basics;

In a problem with a smooth wedge on a smooth plane, a mass is gently placed on the wedge and I've been asked to find the acceleration of the wedge.

Seemed pretty straightforward, just apply F=ma for the wedge?

NO

The answer was obtained by applying F=ma for the particle parallel to the wedge and for the whole system horizontally.

Why couldn't I get the answer by just applying F=ma for the wedge?