if you made the direction down the slope negative ...
-98 + f = 20(-3)
f = 38N
once again ... please post physics problems in the Math Topics forum.
A body of mass 20 kg is released from rest at the top of a rough slope which is inclined at 30 degree to the horizontal. If the body accelerates down the slope at 3m/s^2, find the resistance to motion experienced by the body. ( Assume this resistance to be constant throughout)
My answer: Forceof body= mg= 20x9.8
= 196 N at 30 degree
Force parallel to the plane = 196x sin30= 98N
Fnet= Fp-Fr= ma
98-Fr= 20x-3
--Fr= -60-98
= -158N
Please review and correct me. Thank you very much for your time and help.