According to Newton's third law of motion, for every action, there is an equal and opposite reaction. However, it's important to note that the forces involved act on different objects. Let's analyze the scenario you mentioned:
If you throw an object with a force of 20 Newtons, the object will experience a force of 20 Newtons in the direction you throw it. As a result, the object will accelerate in that direction.
According to Newton's third law, there will indeed be an equal and opposite reaction. However, the reaction force will not directly affect you in the way you described. When you throw the object, your hand exerts a force on it, but your body is braced against the ground or held steady by other means. The reaction force you experience is due to the contact between your body and the ground (or other supporting surfaces), not directly from the force you exert on the object.
So, in this situation, you spend the energy of 20 Newtons to throw the object. The reaction force you experience will depend on the contact forces with the supporting surfaces involved and may not necessarily be 20 Newtons.
It's important to remember that forces occur in pairs according to Newton's third law, but they act on different objects and can have different effects depending on the context.