1. 5 Newton’s Third Law of Motion
Force interactions & Newton’s 3rd Law
Systems
Vectors, Components & Vector Addition
Homework:
RQ: 4, 5, 10, 11, 12, 13, 15, 17, 20, 21.
Ex: 13, 22, 30, 38, 39, 43, 44, 45, 48.
Problems: 1, 2, 3, 4.

2. Forces and Interactions
inter = between
all forces are interactions between masses
each mass feels same size force

3. Newton’s 3rd Law of Motion
Whenever one object exerts a force on a second object, the second object exerts an equal and oppositely directed force on the first.

4. Systems
(net-external force on a system) = (mass of system)x(acceleration of system).
often defined so unknown forces can be determined, e.g., tension can be determined below.

5. What external forces act on this system? Can the system move?

6. What forces act on the man? Draw a Force Diagram for him.

7. Summary of Newton’s Laws
1st: Fnet = 0  constant velocity
2nd: Fnet = ma
3rd: Forces occur in pairs (equal size, opposite direction)

8. Newton’s 3rd Law Pair Notation
use “x” marks on forces that are 3rd Law pairs.
Remember: There will never be two “x” marks on one motion diagram.
The other “x” will be on force diagram of other object.
Use “xx” for a different interaction, etc.

9. Force Diagram each object. Which has greater acceleration when released?

10. Pract. Physics: Page 21 #2(at bottom)

11. Vector Addition
addition is done tail-to-head, also called “parallelogram rule”
added to determine net force
if net force is known, we use vector addition to determine components, e.g. lantern hanging from two ropes. //

12. Which strings have greater tension?

14. Add Velocities to Find Speed & Direction

15. Diagonal of Square is 1.41 times its sides

16. 3-4-5 Rule

17. Components of Vectors component = part
horizontal part is the “x component”
vertical part is the “y component”

18. Practical Vector Addition

19. Summary All forces are interactions, i.e., part of a pair of forces
Newton’s 3rd Law: pairs have equal size opposite direction
Systems are defined to determine unknown forces
Vectors, Vector Components, Parallelogram Rule

20. Practicing physics Do Page 27
Refer to page 26 to review parallelogram rule of vector addition

21. Action and Reaction