Of course I am sure this is not a paradox but rather a problem in my understanding of the situation. But this question is bogging me, and I haven't found a satisfying explanation yet. Suppose you are floating in space, in an inertial frame. An object much more massive than you is moving with constant velocity at your arm's reach. You grab hold of the object, moving along with it. From the perspective of someone staying in the first inertial frame, the massive object transmitted a small part of its kinetic energy to accelerate you to its speed, and energy is conserved. From your perspective, you are not accelerating, instead the object deccelerates until its velocity is zero. It must have lost an enourmous amount of kinetic energy. The only force which could have done any work on this object is your hand's contact force, but (unless you are Hercules) this force is clearly not enough to stop this object. So how has all this energy been lost? I have a possible explanation, which is that when we grab hold of the object, we temporarily enter in a non-inertial reference frame, in which Newton's second law needs a new fictitious force term. This fictitious force will do the major part of the work on the object, and then will vanish when the object stops. But I am not sure what to think of forces appearing and disappearing at any instant.