5%20Newton

1
5 Newtons Third Law of Motion

• Force interactions Newtons 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
Newtons 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.

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What external forces act on this system? Can the
system move?
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What forces act on the man?Draw a Force Diagram
for him.
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Summary of Newtons Laws

• 1st Fnet 0 ? constant velocity
• 2nd Fnet ma
• 3rd Forces occur in pairs (equal size,
  opposite direction)

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Newtons 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.
• //

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Which strings have greater tension?
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(No Transcript)
14
Add Velocities to Find Speed Direction
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Diagonal of Square is 1.41 times its sides
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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
• Newtons 3rd Law pairs have equal size opposite
  direction
• Systems are defined to determine unknown forces
• Vectors, Vector Components, Parallelogram Rule

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Practicing physics

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

21
Action and Reaction