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26-Apr-15 Physics 1 (Garcia) SJSU Chapter 5 Newton’s Third Law of Motion For every “action” there is a “reaction”

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Presentation on theme: "26-Apr-15 Physics 1 (Garcia) SJSU Chapter 5 Newton’s Third Law of Motion For every “action” there is a “reaction”"— Presentation transcript:

1 26-Apr-15 Physics 1 (Garcia) SJSU Chapter 5 Newton’s Third Law of Motion For every “action” there is a “reaction”

2 26-Apr-15 Physics 1 (Garcia) SJSU Newton’s Third Law of Motion Whenever an object exerts a force on a second object, the second object exerts a force of equal magnitude in the opposite direction on the first object. First Object (Hammer) Second Object (Nail)

3 26-Apr-15 Physics 1 (Garcia) SJSU Check Yourself A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h.. The force of impact is greatest on which vehicle, the car or the truck? The change in velocity (the acceleration) is greatest for which vehicle? By what principle of physics?

4 26-Apr-15 Physics 1 (Garcia) SJSU Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greater force exerted on the rope is by 1. Arnold. 2. Suzie. 3. Neither. The force is the same. Check Yourself

5 26-Apr-15 Physics 1 (Garcia) SJSU N, N, or 3. Zero? Does the scale read: 10 Demo: Pulled in Both Directions

6 26-Apr-15 Physics 1 (Garcia) SJSU Demo: Equal Magnitudes With spring scales, we verify that action and reaction forces have equal magnitudes Action Hold Pull Reaction

7 26-Apr-15 Physics 1 (Garcia) SJSU Action and Reaction Common expression of 3 rd Law is, To every action there’s an equal and opposite reaction. What’s an “action”? How can reaction be “equal” and “opposite”?

8 26-Apr-15 Physics 1 (Garcia) SJSU Action-Reaction Pairs Here are some examples of action-reaction pairs. Think of examples of an object exerting a force on a second object. Practice identifying action-reaction pairs. ????

9 26-Apr-15 Physics 1 (Garcia) SJSU Demo: Mutual Attraction What happens when: Mr. A pulls, Mr. B holds. Mr. A holds, Mr. B pulls. Mr. A & Mr. B both pull. Mr. B Mr. A Mr. B has more mass than Mr. A

10 26-Apr-15 Physics 1 (Garcia) SJSU Demo: Mutual Attraction (cont.) If only Mr. A pulls on Mr. B then Mr. B accelerates. Reaction force of equal magnitude so Mr. A also moves. Who moves faster? Mr. A, Mr. B, or the same? Action Reaction Mr. A Mr. B Mr. B has more mass than Mr. A

11 26-Apr-15 Physics 1 (Garcia) SJSU Demo: Mutual Attraction (cont.) When both guys pull then there are two action forces and two reaction forces. If both pull with same force, how much greater is the acceleration than when only one pulls? Action Reaction Mr. A Mr. B Reaction Action

12 26-Apr-15 Physics 1 (Garcia) SJSU Demo: Mutual Repulsion Similar demonstration is to have Mr. A and Mr. B push away instead of pull together. Same results; if Mr. A pushes and Mr. B holds then both move apart. Standing on skateboards Action Reaction Mr. A Mr. B

13 26-Apr-15 Physics 1 (Garcia) SJSU Walking, Running & Jumping What forces accelerate us into motion when we walk, run, or jump?

14 26-Apr-15 Physics 1 (Garcia) SJSU Forces when Walking or Jumping For a person walking, running, or jumping, the three main forces on the person are: Gravity (Downward) Support of the floor (Upward) Frictional force of the floor (Horizontal) Only these forces can accelerate the person. Gravity is constant but the force exerted by the floor can increase in reaction to the person exerting a force on the floor.

15 26-Apr-15 Physics 1 (Garcia) SJSU Reaction Action Walking Forward When weight is on back foot it acts by pushing back on the floor. Reaction is the friction of the floor, which pushes your body forward If there were no friction then dancer would fall straight down and could not walk forward

16 26-Apr-15 Physics 1 (Garcia) SJSU Jumping Jumping is done by pushing downward on the ground (action) so the ground pushes upward on you (reaction). How high you jump depends on the force and on the distance over which you apply that force. Can only push while in contact with the ground so squatting helps by increasing distance.

17 26-Apr-15 Physics 1 (Garcia) SJSU IMPORTANT!!! Action force & reaction force NEVER cancel because they act on different objects! Repeat this to yourself over and over again

18 26-Apr-15 Physics 1 (Garcia) SJSU Check Yourself Miss A pushes the car (action); car pushes back on her (reaction). Do these forces cancel? Force on Miss A is to the left; how can she move forward (to the right)? What if floor had zero friction? Miss A Action Reaction Action Reaction Action- Reaction Pairs

19 26-Apr-15 Physics 1 (Garcia) SJSU Check Yourself Miss B also pushes the car; can she move the car by herself? In terms of Newton’s laws, why is this not possible? What other force does Miss B exert on the car besides her hands? Miss B Action Reaction Action Reaction Action- Reaction Pairs

20 26-Apr-15 Physics 1 (Garcia) SJSU Adding Forces When two forces or more forces act in different directions, finding the net force is more complicated. Have to consider the angle for each force.

21 26-Apr-15 Physics 1 (Garcia) SJSU Parallelogram rule Vector Addition Forces are vectors, with magnitude & direction. Object Force A (20 Newtons) Force B (10 N) Net Force A + B (25 Newtons)

22 26-Apr-15 Physics 1 (Garcia) SJSU Demo: Straighten the Line Pull on the line to make it horizontal. 15 pound Bowling Ball Pull HORIZONTAL As the angle gets smaller, must pull much harder.

23 26-Apr-15 Physics 1 (Garcia) SJSU Demo: Straighten the Line (II) 10 Newton Weight 5 N 10 Newton Weight 15 N As the angle gets smaller, must pull much harder.

24 26-Apr-15 Physics 1 (Garcia) SJSU Parallelogram Rule Weight pull PULL! Net Force Net force is the same in both cases but pulling forces different.

25 26-Apr-15 Physics 1 (Garcia) SJSU Nellie Newton hangs motionless by one hand from a clothesline as shown—which is on the verge of breaking. Which side of the line is most likely to break? 1. Left side 2. Right side 3. 50/50 chance of either side breaking Check Yourself Two upward forces must add together to balance Nellie’s weight.

26 26-Apr-15 Physics 1 (Garcia) SJSU Lab: Force Table Practice addition of forces as vectors in the Physics 1L lab using “force tables.” Hang weights and adjust angles until forces balance.


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