1. The problem statement, all variables and given/known data Recently, I have been wondering whether forces are independent of reference frames(inertial and non inertial). From Newton's law we know that the second law is valid only for inertial frames. But what about the non inertial frames. Let ∑F be the net force on an object in an inertial reference frame k. Let 'm' be it's mass and 'a' be it's acceleration in the frame k. 2. Relevant equations By Newton's second law in the frame k, ∑F = m×a 3. The attempt at a solution Acceleration can have different meaning for different reference frames but certainly we can't conclude about the net force using the second law for both the inertial and non inertial reference frames as it's valid for only one of the cases. So do we then take into consideration the fictitious forces for the non inertial cases for the purpose of calculating the net force. How does it explains then? Please help!