Understanding Newton’s 3rd Law of Motion and Action-Reaction

1. Newton’s 3rd Law of Motion Action - Reaction Chapter 6.3

2. Newton’s 2nd Law (a = F/m) • Newton’s second law is responsible for explaining how objects increase or decrease in speed, or change direction. • If the force is increased, the object will accelerate. • If the mass is increased, the object will accelerate more slowly. • When an object changes direction, it is also said to be accelerating. Huh? You will see… • Newton’s 2nd Law can also be used to explain the forces that exist on stationary objects like your weight or the repulsive and attractive forces of two magnets.

3. What is the source of a force? • If you pull on a rope, do you feel the rope pull on you? • Which is the source of the force, you or the rope?

4. Interaction Pairs • All forces exist in pairs (no isolated forces) that are opposite in direction and equal in magnitude (size). • When you kick a ball, you exert a force on the ball while the ball exerts a force on you. • You pull on a door to open it while the door exerts a force on you. • A hammer exerts a force on a nail while the nail exerts a force on the hammer. • When you pull on a rope, it pulls on you.

5. Newton’s 3rd Law of Motion • When one object exerts a force on another object, the second object exerts an equal but oppositely directed force on the first object. • For every action there is an equal but opposite reaction. (Newton’s 3rd Law is sometimes referred to as the “law of action - reaction.”)

6. Free Body Diagram of Newton’s 3rd Law of Motion Fg(book on Earth) Fg = Force of gravity (Book on Earth) “Reaction” = Fg = Force of gravity (Earth on Book “Action” Fg(Earth on book) Note that the forces come in pairs that have symmetry.

7. Action - Reaction Fg (car on Earth) FN (road on car) - - - - - - - - Ff (road on car) Ff (car on road) Fg (Earth on car) FN (car on road) Reaction Action Fg (car on Earth) FN (car on road) Ff (car on road) Fg (Earth on car) FN (road on car) Ff (road on car) = = = Note: All forces come in equal and opposite pairs

8. What is the Reaction Force? • You push on your desk. • The desk pushes on you. • An airplane propeller pushes on the air. • The air pushes on the airplane propeller. • A tennis racquet hits a ball. • A ball hits a tennis racquet. • You push on the floor as you take a step. • The floor pushes back on you.

9. Force of floor on person. Force of person on floor. Do the forces cancel each other out? • No. • Since the forces act on different objects, it is not possible for them to cancel each other out. Ff = mfaf Fp = mpap

10. F2 F1 Newton’s 3rd Law of Motion & Acceleration • Newton’s 3rd Law of Motion says that F1 = F2 • And Newton’s 2nd Law of Motion says that m1a1 = m2a2 • While the forces may be equal but opposite in direction, the accelerations of each object may be different.

11. Example: Newton’s 3rd Law of Motion A physics student is pulling on a rope, which in both cases causes the scale to read 500 Newtons. The physics student is pulling a. with more force when the rope is attached to the wall. b. with more force when the rope is attached to the elephant. c. the same force in each case.

12. FN Fg Newton’s 3rd Law of Motion? • Does the following diagram illustrate an action-reaction pair? • No, because both forces act on the same object. FN = Normal force of Desk on Book = Fg = Force of Gravity on Book

13. Key Ideas • Newton’s 3rd Law of Motion states that when two objects interact, the magnitude of the force exerted by object 1 on object 2 is equal to the magnitude of the force simultaneously exerted by object 2 on object 1, and are in opposite directions. • Newton’s 3rd Law of Motion is also known as the Law of Action-Reaction. • All forces in nature come in pairs. • While the forces are equal and opposite, they do not cancel each other out because the two forces act on two different objects.