Newton’s Laws of Motion Chapter 5

Introduction A) Laws were a summary of previously established relationships and principles concerning force and motion B) Sir Isaac Newton (1642-1727) C) Kinematics: description of motion without considering the causes of motion D) Dynamics: analysis of the causes of motion E) Newton’s Laws summarize the dynamics of motion

Forces and Net Forces A) Objectives: B) What is a Force? 1) to relate forces and motion 2) to explain what is meant by a net or unbalanced force B) What is a Force? 1) a push or a pull on an object 2) a force is something that is capable of producing a change in an object’s state of motion 3) word capable is important…forces can act on an object and not cause it to change it’s motion

Concept of Forces D) if no change in motion occurs, the forces are balanced and their net effect is zero, or the net force is zero E) the vector sum of the forces acting on the object is also zero F)UNBALANCED FORCES CAUSE OBJECTS TO ACCELERATE! G) Contact vs. Field Forces

III. Newton’s First Law A) Objectives 1) to state and explain Newton’s First Law of Motion 2) describe the relationship between inertia and mass B) Aristotle: an objects natural state of motion was to be at rest C) accepted for 1500 years until Galileo did experiments with falling objects and inclined planes D) He determined that objects at rest tended to remain at rest and that objects in motion tended to remain in motion

E) inertia: the natural tendency of an object to maintain a state of rest or to remain in motion in a straight line F) Newton: redefined mass as a measure of an object’s inertia. The more massive an object, the more resistance to changes in motion G) First Law: “In the absence of an unbalanced applied force, a body at rest remains at rest and a body already in motion remains in motion with a constant velocity” H) sometimes referred to as the law of inertia

Newton’s Second Law A) Objectives B) Unbalanced forces….? 1) to state and explain Newton’s second law of motion 2) apply this law to physical situations 3) to distinguish between weight and mass B) Unbalanced forces….? C) ….cause acceleration! D) Newton observed that greater forces were needed to accelerate objects to different speeds F) acceleration was proportional to Forces(net)

F) Newton also observed that greater forces were required to accelerate more massive objects G) acceleration was inversely proportional to mass… a~ 1/m H) Combining the two relationships results in a=f/m or force=mass x acceleration I) second law states: “The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The direction of the acceleration is in the direction of the applied net force.”

J) SI unit for force is the Newton (N) K) kg-m/sec2 L) weight can be considered mass under the influence of gravitational forces M) weight = mass X acceleration ( gravity) N) therefore, it can be said that weight is a force!

IV. Newton’s Third Law A) Objectives 1) to state and explain Newton’s Third Law 2) identify action-reaction force pairs B) Newton recognized that it is impossible to have a single force acting on an object, that all forces act in pairs! C) Third Law: “for every action force, there is an equal and opposite reaction force”

4.5 Applications of Newton’s Laws Free body diagrams are used to identify the forces that act on an object First condition of Equilibrium

4.6 Force of Friction Friction is a resistance to motion For friction to occur: Objects must be in contact Objects must be moving Static vs. kinetic friction Coefficient of static friction

Friction Graph