1. Ch. 5: Newton’s Laws of Motion DYNAMICS

2. Force Force: “A push or a pull”. F is a VECTOR! Vector Addition is needed vector to add Forces!

3. Examples of Forces “Pulling” Forces “Contact” Forces “Pushing” Force “Field” Forces (Physics II):

4. 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

5. Measurement of Forces: Spring Scale Vector addition to add Forces! 

6. 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

7. The 4 Fundamental Forces of Nature The 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)

8. The 4 Fundamental Forces of Nature The Sources of these forces: In order of decreasing strength

9. “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

10. 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

11. 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 common MISCONCEPTION in the 21 st Century! Proven by Galileo in the 1620’s! 

12. 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

13. 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 middle age  Sir Isaac Newton as a young man

14. Newton’s First Law 1 st 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 was born the same year Galileo died! Sir Isaac Newton  as an older man

15. Newton’s 1 st 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 1 st Law: Can define a Force as “An action which causes a change in the motion of an object.”

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

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

18. 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?

19. 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 1 st 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 Law Alternative Statement