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Dynamic problem building an RC robot

We are building a 4 motor driven, with the middle wheels being idle for support. The parameters known are :

Mass= 50 kgs

Theta (Angle ) = 60 degree

Acceleration = 0.254 m/s^2

Assuming coefficient of kinetic friction = 0.6

I am attaching the work I have done and where I get stuck.

I am having trouble calculating the normal forces (N) and frictional force so if someone could help me or tell me what am I missing in the free body diagram or the calculation it would be of great help. I am getting a negative value of N2 which doesn't seem correct. Either way if someone could check and confirm my calculation.

I need to solve this so we can choose a motor for our vehicle, so involves calculation of torque which also I have included.

[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1812&stc=1[/IMG][IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1811&stc=1[/IMG]

Thank you very so much.