1. Chapter 5 Newton’s Third Law of Motion
Print MySJSU roster & add codes. Bring dice for lottery. Bring vampire stake demo. Distribute Greensheet. Distribute survey. Distribute sample solution.
For every “action” there is a “reaction”
12-Apr-17
Physics 1 (Garcia) SJSU

2. 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)
12-Apr-17
Physics 1 (Garcia) SJSU

3. 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?
12-Apr-17
Physics 1 (Garcia) SJSU

4. Check Yourself
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.
Ch 5-5
12-Apr-17
Physics 1 (Garcia) SJSU

5. Demo: Pulled in Both Directions10 10
Ch 5-4
Does the scale read:
1. 10 N,
2. 20 N, or
3. Zero?
12-Apr-17
Physics 1 (Garcia) SJSU

6. Demo: Equal Magnitudes
With spring scales, we verify that action and reaction forces have equal magnitudes.
Action
Reaction 5 15
Pull
Hold 10 10 15 5
12-Apr-17
Physics 1 (Garcia) SJSU

7. Action and Reaction Common expression of 3rd Law is,
To every action there’s an equal and opposite reaction.
What’s an “action”?
How can reaction be “equal” and “opposite”?
12-Apr-17
Physics 1 (Garcia) SJSU

8. 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.
????
????
????
????
12-Apr-17
Physics 1 (Garcia) SJSU

9. 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 has more mass than Mr. A
Mr. B
Mr. A
12-Apr-17
Physics 1 (Garcia) SJSU

10. 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?
Mr. A
Mr. B
Reaction
Action
Mr. B has more mass than Mr. A
12-Apr-17
Physics 1 (Garcia) SJSU

11. 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?
Mr. A
Reaction
Mr. B
Action
Action
Reaction
12-Apr-17
Physics 1 (Garcia) SJSU

12. 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.
Mr. B
Mr. A
Reaction
Action
Standing on skateboards
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Physics 1 (Garcia) SJSU

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

14. 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.
12-Apr-17
Physics 1 (Garcia) SJSU

15. 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
Reaction
Action
12-Apr-17
Physics 1 (Garcia) SJSU

16. 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.
12-Apr-17
Physics 1 (Garcia) SJSU

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

18. 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
Pairs
Action
Reaction
Action
Reaction
12-Apr-17
Physics 1 (Garcia) SJSU

19. 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?
Action-
Reaction
Pairs
Miss B
Action
Reaction
Action
Reaction
12-Apr-17
Physics 1 (Garcia) SJSU

20. 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.
12-Apr-17
Physics 1 (Garcia) SJSU

21. Vector Addition Forces are vectors, with magnitude & direction.
Net Force
A + B
(25 Newtons)
Force B
(10 N)
Parallelogram rule
Force A
(20 Newtons)
Object
12-Apr-17
Physics 1 (Garcia) SJSU

22. Demo: Straighten the Line
Pull on the line to make it horizontal.
HORIZONTAL
Pull
Pull
As the angle gets smaller, must pull much harder.
15 pound
Bowling Ball
12-Apr-17
Physics 1 (Garcia) SJSU

23. Demo: Straighten the Line (II)
As the angle gets smaller, must pull much harder.
5 N
5 N
15 N
15 N
10 Newton
Weight
10 Newton
Weight
12-Apr-17
Physics 1 (Garcia) SJSU

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

25. Check Yourself
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
Two upward forces must add together to balance Nellie’s weight.
5-11
12-Apr-17
Physics 1 (Garcia) SJSU

26. 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.
12-Apr-17
Physics 1 (Garcia) SJSU