2. Newton’s 3 rd Law Newton’s 2 nd Law: A quantitative description of how forces affect motion BUT: Where do forces come from? EXPERIMENTS show that Forces applied to an object are ALWAYS applied by another object  Newton’s 3 rd Law “ Whenever one object exerts a force F on a second object, the second object exerts an equal & opposite force -F on the first object.” –The “Law of Action-Reaction” “Every action has an equal & opposite reaction”. (Note that action-reaction forces act on DIFFERENT objects!)

3. Another Statement of Newton’s 3 rd Law “If two objects interact, the force F 12 exerted by object 1 on object 2 is equal in magnitude & opposite in direction to the force F 21 exerted by object 2 on object 1.” As in the figure

4. Example: Newton’s 3 rd Law When a force is exerted on an object, that force is caused by another object. Newton’s 3 rd Law: “Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.” If your hand pushes against the edge of a desk (the force vector shown in red), the desk pushes back against your hand (this force vector is shown in purple, to remind us that this force acts on a DIFFERENT object).

5. Newton’s 3 rd Law: Alternative Statements 1. Forces always occur in pairs 2. A single isolated force cannot exist 3. The “action force” is equal in magnitude to the “reaction force” & opposite in direction a. One of the forces is the “action force”, the other is the “reaction force” b. It doesn’t matter which is considered the “action” & which the “reaction” c. The action & reaction forces must act on different objects & be of the same type

6. Action-Reaction Pairs: Act on Different Objects The key to correct the application of Newton’s 3 rd Law is: The forces are exerted on different objects. Make sure you don’t use them as if they were acting on the same object. Example: When an ice skater pushes against the railing, the railing pushes back & this reaction force causes her to move away.

7. Rocket propulsion can be explained using Newton’s Third Law: Hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket. Note: The rocket doesn’t need anything to “push” against.