1. For every force, there is an equal and opposite force.

2.  Define force as part of an interaction (7.1)  State Newton’s third law of motion (7.2)  Describe how to identify a pair of action reaction forces (7.3)  Explain why the accelerations caused by an action force and by a reaction force do not have to be equal (7.4)  Explain why an action force is not canceled by a reaction force(7.5)  Explain how a horse-cart system accelerates (7.6)  Explain what must occur in every action between things (7.7)

3. 7.1 Forces and Interactions  A force is always part of a mutual action that involves another force  The interaction that drives the nail is the same that stops the hammer  A mutual action is an interaction  Force is an interaction between one object and another

4. 7.2 Newtons Third Law  Newtons third law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object  When the girl jumps to shore, the boat moves backward

5. 7.3 Identifying Action and Reaction  To identify a pair of action-reaction forces, first identify the interacting objects A and B, and if the action is A on B, the reaction is B on A  Action: object A exerts a force on object B  Reaction: object B exerts a force on object A

6. 7.4 Action and Reaction on Different Masses  Apply newtons third law to tug-of-war. If the action is you pulling on the rope, is the reaction force the ground pushing back on you or your opponent pulling back on the rope?  Neither the reaction force is the rope pulling back on you, A on B; B on A

7. 7.4 – continued  A given force exerted on a small mass produces a greater acceleration than the same force exerted on a large mass  The larger the letter for the variable, the larger the magnitude that variable has  F F m  An action -reaction force does not need a medium to occur, these forces can occur in a vacuum – like outer space  When the engines of an airplane push the air backwards, the air pushes the plane forward and the shape of the wings force the air down to create lift = m = a a

8. 7.5 Defining Systems  Action and reaction forces do not cancel each other when either of the forces is external to the system being considered  So if two people with the same mass push off each other on ice, they will move backwards the same distance

9. 7.6 The Horse-Cart Problem  The forces don’t actually equal zero; it depends on perspective.  We need to look at the different systems involved in the problem  3 different systems  Horse  Cart  Horse and cart  The net force on the cart divided by the mass of the cart will produce an acceleration

10.  Looking at the horse, the cart does restrain the horse but the horse can push on the ground and overcome the force of the cart  if the horse and cart were in equilibrium then as the horse walks the cart follows.  You can’t get a car moving by sitting in the car and pushing on the dash board you must get out and push on the ground and the car. 7.6 The Horse-Cart continued

11. ForceByOnDirection Affects the Motion of Comments Horse pulls wagon HorseWagonRightWagon Action/Reaction WagonHorseLeftHorse Horse pushes Ground HorseGroundLeftGround Action/Reaction GroundHorseRightHorse Friction GroundWagonLeftWagon Action/Reaction WagonGroundRightGround

12. Help on figuring out action- reaction forces  Difficulties that occur with the action-reaction situations usually stem from failing to clearly identify the system in question.  Basically, if you want to know the effect of a force or forces on something, call that something your system. 1. Define your system by a real or imaginary dotted line around that something. 2. Restrict your attention to the external forces that originate outside the dotted line and act on the system, and NOT to the forces that the system may exert on things external to the dotted line

13. 7.7 Action Equals Reaction  For every interaction between things, there is always a pair of oppositely directed forces that are equal in strength