Ok so I have this problem which I wasn't really sure how to approach; when I looked at the results I understood that it needed newton's 3rd law to be solved and I sort of made it up, but I don't understand why it turns out to be correct(i only have the results, no explanation) and I feel like I've been misusing the law. 1. The problem statement, all variables and given/known data A platform of mass m2 = 4 kg is at rest over a frictionless horizontal plane; a body of mass m1 = 2 kg is at rest over the platform and there is no friction. This body starts moving, due to an internal motor, to the right with an acceleration a = 6.3 m/s2 with respect to the platform. Calculate the accelerations of m1 and m2 with respect to the plane. 2. Relevant equations Newton's 2nd and 3rd law; relative acceleration: a = a' + at where a is the acc in the inertial fixed frame, a' the acc in the moving frame, and at the acc of the moving frame with respect to the fixed one 3. The attempt at a solution I'll now call a1 the acceleration of m1 relative to the fixed frame and a2 the acceleration of m2 relative to the fixed frame In the fixed frame there is a force F1 acting on m1 such that F1 = m1a1 from the relative acc. law: a1 = a + a2 ⇒ F1 = m1(a + a2) due to newton's 3rd law the resultant of internal forces of the system must be 0, so there should be a force F2 acting on m2 which is opposite to F1: -F1 = F2 = m2a2 Now i get -m2a2 = m1(a+a2) ⇒ -a2(m2+m1) = m1a ⇒ a2 = -m1a/(m1+m2) = -2.1 m/s^2 (negative so m2 is pushed to the left) a1 = a + a2 = 4.2 m/s^2 So, what I don't get is, why does newton's 3rd law apply here? I went back to check how my theory book explains it and it talks about points exerting a force on each other, but here how is m1 exerting a force on m2 through his motion? If I didn't look at the solutions I would have never thought about this way of solving it. I am pretty sure the key is in the fact, already noted, that the force applied is internal to the system, but I still don't understand D: after all they are two separate bodies and there is no friction between them, so how is this working exactly? Is there some real-world situation that can help me visualize that or some other ways I can think about it?