2. Newton’s Third Law The Most Misunderstood Concept in Science

3. F net = MA The ACCELERATION of an object is directly proportional to the NET FORCE (unbalanced force) acting on it and is inversely proportional to its MASS where the direction of the ACCELERATION is in the direction of the unbalanced force. L A W # 1 L A W # 2 An object at rest will continue to stay at rest or will continue to travel at a constant speed in a straight line unless compelled to change by forces acting on it. F net = 0 “Law of Inertia”

4. L A W # 3 When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. “For every action there is an equal and opposite reaction” “Rule of Forces: ALL forces occur in PAIRS where F 1 =-F 2 ”

5.  Law #1: All forces on object balanced  Law #2: All forces are NOT balanced  All motion is either Law #1 OR Law #2  ALL motion is Law #3

6. Law #1#2 Law #1 and #2 deals with forces acting ON A SINGLE OBJECT Law #3 Law #3 deals with forces BETWEEN TWO OBJECTS

7. F net = MA The acceleration of an object is directly proportional to the net force (unbalanced force) acting on it and is inversely proportional to its mass where the direction of the acceleration is in the direction of the unbalanced force L A W # 1 L A W # 2 An object at rest will continue to stay at rest or will continue to travel at a constant speed in a straight line unless compelled to change by forces acting on it F net = 0 “Law of Inertia” Two where the forces acting between the object occur in equal and opposite pairs.

8. HW #4-1 (III) page 130 [MC] #9,10,11 #9. The action reaction forces of Newton’s third law (a) are in the same direction, (b) have different magnitudes, (c) act on different objects, (d) can be the same force.

9.  Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other. The effect of the force is not always the same.

10. Force (on bullet)= -(Force (on gun)) F=ma (for both) m bullet a bullet = - (m gun a gun )

11. #10. A brick hits a glass window. The brick breaks the glass, so (a) force of brick greater than force of glass, (b) force of brick is smaller than force of glass, (c) force of brick equals force of glass, (d) none of these

12. #11. A freight truck collides head on with a passenger car causing a lot more damage to the car than the truck. Which is true? (a) The force of the truck on the car is bigger than the force of the car on the truck. (b) The force of the truck on the car is smaller than the force of the car on the truck. (c) The force of the truck on the car is the same as the force of the car on the truck. (d) None of the above

13. CQ #13. Here is a story of a horse and a farmer… One day, the farmer attaches a heavy cart to the horse and demands that the horse pull the cart. “Well,” says the horse, “I can’t pull the cart because, according to Newton’s Third Law, if I apply a force to the cart, the cart will apply an equal and opposite force on me. The net result will be that I cannot pull the cart, since all the forces will cancel.” The farmer was very upset. What is wrong with the horse’s reasoning?

14. yellow The two forces colored yellow in the diagram are a Newton's Third Law force pair - "horse pulls wagon" and "wagon pulls horse". They are equal in magnitude and opposite in direction. The two forces colored blue in the diagram are a Newton's Third Law force pair - "horse pushes ground" and "ground pushes horse". They are also equal in magnitude and opposite in direction. *only the forces ON the HORSE cause the horse to accelerate.

15. [E] #31, 32 #31. A book is sitting on a horizontal surface. How many forces are acting on the book? TWO: Gravity and the table

16. B B #32. In an Olympic figure skating event, a 65-kg skater (B) pushes a 55-kg skater (A), causing her to accelerate at 2.0 m/s 2. At what rate will skater B accelerate? FA=FBFA=FB mAaA=mBaBmAaA=mBaB (55 kg)(2.0 m/s 2 )=(65 kg)( a B ) 110 N /65 kg= a B 1.7 m/s 2 = a B

17. How does a rocket fly (according to Newton’s Laws)?