A bird flies by pushing downwards the air below its wings with a force. According to Newton's third law, the air pushes the wings upwards. As a result, the bird is lifted. Also consider the case of a rocket. The fuel it ejects exerts an upward force and as a result, the rocket moves upwards. The second law states that vector sum of the all the forces acting on the particle is given by the product of its mass and acceleration. In the above cases, should they have acceleration as the action force shouldn't be included in the "net " force "acting" on them?Is it the reaction force causes them to accelerate? Another question. I am standing on the ground. I am exerting a force on the ground and ground of course by the 3rd law exerts a normal force.If I consider only normal force as the force acting on me, then according to 2nd law,I should have an acceleration right? I am bit confused whether I take gravitational force as it is the one which causes us to exert a force on the ground.