Identify the action and reaction forces? a. A hammer hits a nail. b. Earth's gravity pulls down on a book. c. A helicopter blade pushes air downward.

 

Answer

a) action: the force applied by the hammer reaction: the nail applies an equal, in magnitude but opposite in direction, force to the hammer. b) action: gravity pulls the book down reaction: the book "pulls the earth up" <--yes this really happens. But the force is equal to the force required to pull the book down, so divide that force by the mass of the earth and you will see that the acceleration applied by the book on the earth is almost zero. That's why you don't notice this effect. c) action: blade pushes air down reaction: air pushes helicopter blade up <--which is what makes helicopters fly. In all action reaction forces, first look at what force is being applied, like in (a) obviously the action force is the hammer driving the nail. Now, to find the reaction force, just remember that the action and reaction forces NEVER are applied to the same object. The object that applies the action force, is the object that receives the reaction force, similarly the object that receives the action force is the object that applies the reaction force.