Physics Announcements Clickers (should be in Mon or Tues) WebAssign – must have Account Codes in 2 weeks (from yesterday) –You can use without a code till then –Homework opens on the day of class, closes 1 week later before class. Exam #1 Wednesday –Covers Chapters 1-4

Class Schedule 1/17Chapter 1Introduction, About Science 1/22Chapter 2Newton’s First Law 1/24Chapter 3Linear Motion 1/29Chapter 4Newton’s Second Law 1/31Midterm Exam 1 2/5Chapter 5Newton’s Third Law

Chapter 4 Newton’s Second Law Force Causes Acceleration Friction Mass and Weight Mass Resists Acceleration Newton’s Second Law Free Fall – When Acceleration = g Nonfree Fall – When Acceleration < g

Force Causes Acceleration According to Newton’s First Law: - An object can’t accelerate unless a force acts on it Examples: Stationary object.. Won’t move unless pushed Cart on Air Track.. Moves at constant velocity unless stopped or pushed - Acceleration depends on the NET force on an object. - Double the net force; double the acceleration - Triple the net force; triple the acceleration Acceleration ~ Net Force (means … “is proportional to”) … in other words … The Physics Place … video “Force Causes Acceleration”

Friction The force of friction always opposes the direction of motion Friction occurs when surfaces slide against each other. Friction Applet: Inclined Plane Friction Applet: Traffic Jam Static Friction: Resists STARTING something in motion. Sliding Friction: Resists motion once it has started. Force of friction doesn’t depend on the AREA in contact … it only depends on the mass of the object. Friction in air = air resistance (or air drag)

Mass and Weight Mass: a measure of the inertia of an object. Weight: The force exerted on an object by gravity. Weight = Mass x acceleration of gravity W = mg Units of mass = kg English Units of weight = pounds (lb) A brick with a mass of 1kg weighs 2.2 lb In metric units, weight is expressed in Newtons (N) The acceleration of gravity is g = 9.8 m/s 2 (or about 10 m/s 2 ) A brick with a mass of 1kg weighs 9.8 N (or about 10 N) Problem: What is the weight of a ball with a mass of 2 kg? W = (2 kg)(10 m/s 2 ) = 20 N

Question 1 What is the weight of a 1 kg brick? a.1 kg. b.2.2 lb. c.11 N. d. 100 carats.

Mass Resists Acceleration Heavier objects are harder to accelerate! So the amount of acceleration you can impart to an object depends on the mass of the object. The less mass, the more acceleration. The more mass, the less acceleration. Acceleration ~ 1 mass or

Newton’s Second Law The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. In other words …. F = ma Remember this equation!!!

Question 2 Which of the following is a statement of Newton’s Second Law? a.E = mc 2 b.What goes around comes around. c.F = ma d. Haste makes waste.

Total Acceleration = g … Free Fall When the force of gravity is the only force acting on an object. No supporting force. No air resistance. Net force = the object’s weight (Weight = mg) Force = maThe acceleration of an object in free fall is g Example: The net force on a 10N object in free fall is 10N Free fall animation – elephant vs. feather

Total Acceleration < g … non-Free Fall In the presence of air resistance, the net force acting on a falling object is less than the object’s weight! Non-free fall animation – elephant vs. feather Net Force = 1000N Down Net Force = 600N Down Net Force = 200N Down Net Force = 0 a = Net Force mass Weight – air resistance mass =