5 Newton’s Third Law of Motion
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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.
1
Forces and Interactions
• inter = between
• all forces are interactions between masses
• each mass feels same size force
2
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.
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
• 1st:
• 2nd:
• 3rd:
Fnet = 0  constant velocity
Fnet = ma
Forces occur in pairs (equal size,
opposite direction)
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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.
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Force Diagram each object. Which
has greater acceleration when
released?
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Pract. Physics: Page 21 #2(at
bottom)
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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?
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Add Velocities to Find Speed & Direction
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Diagonal of Square is 1.41
times its sides
15
3-4-5 Rule
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Components of Vectors
• component = part
• horizontal part is the “x component”
• vertical part is the “y component”
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Practical Vector Addition
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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
19
Practicing physics
• Do Page 27
• Refer to page 26 to review parallelogram
rule of vector addition
20
Action and Reaction
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