Why can't I find wedge acceleration using only F=ma for the wedge?

[AStudentNeedsHelp]

↪ mentor note: thread moved to h/w forum so template is missing ↩

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?

 

[NascentOxygen]

Does the question ask for the relative horizontal acceleration? The wedge moves one way, and the body moves the other way.

 

[SammyS]

↪ mentor note: thread moved to h/w forum so template is missing ↩

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?

How do you propose finding the net force acting on the wedge ?

 

[OmneBonum]

You should probably use the homework template as per the guidelines. I'm not sure you've given enough information about the problem for anyone to be able to help you.