Lesson Five – Forces Newtons laws The third law

Action and reaction “For every action force, there is an equal and opposite reaction force” This is the hardest of Newtons laws to understand. Let’s start with some easy examples….

Action and reaction - Dynamic

Action and reaction - Dynamic

Action and reaction - Dynamic

Action and reaction – static contact forces

Action and reaction – static contact forces

Action and reaction – static contact forces

Action and reaction – static contact forces

Action and reaction In dynamic situations the action and reaction forces act on DIFFERENT OBJECTS so that an acceleration occurs In static situations the action and reaction forces will act on the SAME OBJECT, cancelling out. Even in simple situations there may be many pairs of action and reaction forces.

Action and reaction What are the forces working here?

Action and reaction Gravity

Action and reaction Gravity Air resistance Are these an action and reaction pair?

Action and reaction No! at the start of the motion , they are NOT equal! So where is the reaction force to gravity on the skydiver?

Action and reaction As the earths gravity acts on the skydiver, the skydiver’s gravity acts on the earth, pulling the earth up toward him! Why don’t we notice this?