Ch. 5: Newton’s Laws of Motion DYNAMICS

Concept of Force Force: “A push or a pull”. F is a VECTOR! Need vector addition to add forces! Vector addition to add Forces!

“Field” forces (Physics II) Examples of Forces “Pushing” forces “Contact” forces “Pulling” forces “Field” forces (Physics II)

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

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

Inertial Reference Frame 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 Newton: Built on Galileo’s work 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

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 was born the same year Galileo died! Sir Isaac Newton  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 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!