Ch. 5: Newton’s Laws of Motion DYNAMICS

Force Force: “A push or a pull”. F is a VECTOR!

Vector Addition is needed vector to add Forces!

Examples of Forces “Pulling”

Forces“Contact” Forces

“Pushing” Force

“Field” Forces (Physics II):

• 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

Measurement of Forces: Spring Scale

Vector additionto addForces!

• 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

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)

The 4 Fundamental Forces of NatureThe Sources of these forces: In order of decreasing strength

• “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

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

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!

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

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

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

• 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.”

• Newton’s 1st Law: First stated by Galileo!

Newton’s First Law

A Mathematical Statement of Newton’s 1st Law:If v = constant, ∑F = 0 OR if v ≠ constant, ∑F ≠ 0

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?

• 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