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Ch. 5: Newton’s Laws of Motion DYNAMICS
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Force Force: “A push or a pull”. F is a VECTOR!
Vector Addition is needed vector to add Forces!
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Examples of Forces “Pulling”
Forces“Contact” Forces
“Pushing” Force
“Field” Forces (Physics II):
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• Contact forces involve physical contact between two objects– Examples (in pictures):
spring force, pulling force, pushing force
• Field forces act through empty space.– No physical contact is required.
– Examples (in pictures):
gravitation, electrostatic, magnetic
Classes of Forces
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Measurement of Forces: Spring Scale
Vector additionto addForces!
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• Gravitational Forces– Between objects
• Electromagnetic Forces– Between electric charges
• Nuclear Weak Forces– Arise in certain radioactive decay processes
• Nuclear Strong Forces– Between subatomic particles
Note: These are all field forces!
Fundamental Forces of Nature
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The 4 Fundamental Forces of NatureThe Sources of these forces: In order of decreasing strength
• Strong Nuclear Force: – Binds nuclei together. Still being researched.
• Electromagnetic Force:– E&M phenomena. Chemical forces. Most everyday
forces. Maxwell, Coulomb, Ampere, Faraday, ...• Weak Nuclear Force:
– Nuclear decay. Fermi, Bethe & others. Still being researched.
• Gravitational Force:– Newton (“classical” mechanics)– Einstein (general relativity)
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The 4 Fundamental Forces of NatureThe Sources of these forces: In order of decreasing strength
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• “Electro-Weak” Force: – Since ~ the late 1960’s, in some sense have
reduced the 4 fundamental forces to 3!
– The Electromagnetic Force & the Weak Nuclear Force were combined into one theory.
– S. Weinberg & A. Salaam: The 1972 Nobel Prize in Physics!
The 4 Fundamental Forces of Nature
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Sir Isaac Newton• 1642 – 1727• Formulated the
Basic Laws of Mechanics• Discovered the
Law of Universal Gravitation
• Invented a form of
Calculus• Made many observations dealing
with Light and Optics
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Newton’s Laws of Motion • The ancient (& wrong!) view (of Artistotle):
– Need a force to keep an object in motion.– The “natural” state of an object is at rest.
• The CORRECT VIEW (of Galileo & Newton):– It’s just as natural for an object to be in motion at constant
speed in a straight line as to be at rest.– At first, imagine the case of NO FRICTION
– Experiment: If NO FORCE is applied to an object moving at a constant speed in straight line, it will continue moving at the same speed in a straight line! If I succeed in having you overcome the wrong ancient misconception & understand the correct view of this,
A MAJOR GOAL of the COURSE WILL HAVE BEEN ACHIEVED!
A commonMISCONCEPTIONin the 21st Century!
Proven by Galileo
in the 1620’s!
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Reference Frames Inertial Reference Frame
(As defined by Newton)
≡ A reference frame (coordinate system) which is moving with constant velocity
(no acceleration!) with respect to the “fixed stars”.
– Clearly, an idealization!
Rigorously, Newton’s Laws are ONLY valid in an Inertial
Reference Frame
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Newton’s Laws• Galileo laid the ground work for Newton’s Laws.
• Newton: Built on Galileo’s work
• Newton’s 3 Laws: One at a time
Galileo Galilei in middleage
Sir Isaac Newton as a young man
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Newton’s First Law• 1st Law: (“Law of Inertia”): “In the absence of
external forces and when viewed from an inertial reference frame, an object at rest remains at rest and an object in motion remains in motion with a constant velocity (constant speed in a straight line).”
Newton wasborn the sameyear Galileo
died!
Sir IsaacNewton as an older man
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• Newton’s 1st Law: (Alternate Language):
1. “When no net force (∑F = 0) acts on an object, the acceleration of the object is zero.”
∑ = a math symbol meaning sum (capital sigma)
2. “If an object does not interact with other objects, it is always possible to identify a reference frame (an inertial frame) in which the object has zero acceleration.”
• From the 1st Law: Can define a Force as “An action which causes a change in the motion of an object.”
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• Newton’s 1st Law: First stated by Galileo!
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Newton’s First Law
A Mathematical Statement of Newton’s 1st Law:If v = constant, ∑F = 0 OR if v ≠ constant, ∑F ≠ 0
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Conceptual Example
Newton’s First Law
A school bus comes to a sudden stop, and all of the backpacks on the floor start to slide forward. What force causes them to do that?
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• In the absence of external forces, when viewed from an inertial reference frame, an object at rest remains at rest & an object in motion continues in motion with a constant velocity.– Newton’s 1st Law describes what happens in
the absence of a net force.– It also tells us that when no force acts on an
object, the acceleration of the object is zero.
Newton’s First LawAlternative Statement