 Forces cannot exist individually  Any force applied to an object causes an interaction  And the object applies an equal force back

 Whenever one object applies a force to another, the other applies and equal and opposite force  Action & reaction forces  Always occur in pairs

 First find out where the interaction is occuring  It does not matter what the “action” and “reaction” is, the are the same size

 If a boulder falls to earth (because of an action force), does the earth fall to the boulder?  Recall: accelerations depend upon two things according to 2 nd law: force applied and mass  Since the force on the boulder and the earth is the same, the only other variable is mass  The earth has huge inertia, therefore accel. Is very small

 Same sort of thing happens with cannons and guns  Rocket propulsion is an action/reaction pair  Air applies an action force and accel out  The reaction force is the rocket accel in opposite direction

 Planes and helicopter fly because of a reaction force called lift  The blades and wings push (force) the air downward  The air reacts by pushing upwards