5 Newtons Third Law of Motion Force interactions

5 Newton’s Third Law of Motion • • Force interactions & Newton’s 3 rd 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. 1

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

Newton’s 3 rd 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. 3

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. 4

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

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

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

Newton’s 3 rd Law Pair Notation • use “x” marks on forces that are 3 rd 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. 8

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

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

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. • // 11

Which strings have greater tension? 12

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Add Velocities to Find Speed & Direction 14

Diagonal of Square is 1. 41 times its sides 15

3 -4 -5 Rule 16

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

Practical Vector Addition 18

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

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

Action and Reaction 21