Unit 4: Forces and Newton’s Laws of Motion Why physics teachers don’t monitor recess

I. Force Equation FORCES CAUSE ACCELERATION Equation (see Reference Tables): Example: What net force is required to accelerate a 1500 kg race car at + 3 m/s2?   FORCES CAUSE ACCELERATION

II: Weight vs. Mass MASS IS A CONSTANT MEASUREMENT Then what is its acceleration? Equation (see reference tables): Example: Find the weight of a 35 kg mass on Earth.   WEIGHT IS KNOWN AS THE FORCE DUE TO GRAVITY a = g = 10 m/s2 (ON EARTH)

III. Normal Force and Free Body Diagrams Forces acting on a book Forces acting on a book resting resting on a flat desk on a desk at an angle   FORCE FROM THE SURFACE PUSHING BACK ON OBJECT ALWAYS ACTS PERPENDICULAR TO THE SURFACE

IV: Apparent Weight (REST OR CONSTANT VELOCITY) Example: An elephant with a mass of 1000 kg is standing on a scale inside an elevator that is at rest. What is the scale reading (apparent weight of elephant)?   THE NORMAL FORCE = WEIGHT WHEN ON A FLAT SURFACE IN EQUILIBRIUM (REST OR CONSTANT VELOCITY)

IV: Apparent Weight   What happens to your “weight” if the elevator moves? Video (3 min) Elevator Safety – Bill Nye video (4 min Start at 2:30) Nellie in the Elevator (3 min)

V. Change in “Weight” Example: The elevator from the above example begins to move. A) What would happen to the scale reading if the elevator accelerates up?   THE NORMAL FORCE (APPARENT WEIGHT) CAN CHANGE IF THE OBJECT IS ACCELERATING

V. Change in “Weight” Example: The elevator from the above example begins to move. B) What would happen to the scale reading if the elevator accelerates down?  

V. Change in “Weight” Example: The elevator from the above example begins to move. C) What would happen to the scale reading if it was moving at a constant speed up?  

Part B: Weight vs. Mass V. Change in Weight Aboard the Vomit Comet video (4 min)   What happens to your “weight” if the elevator moves? Video (3 min) Elevator Safety – Bill Nye video (4 min Start at 2:30) Nellie in the Elevator (3 min)

Part C: Newton’s Laws of Motion VI. Newton’s First Law of Motion Inertia: Units: Examples: 1. Which object has more inertia: 2 kg pumpkin moving at 20 m/s or 10 kg turkey at rest?    2. What happens to the inertia of an object, if the mass increases? TENDENCEY FOR AN OBJECT TO REMAIN AT REST OR CONSTANT VELOCITY (a = 0) (INTERIA = MASS) kg 10 kg TURKEY (MORE MASS) INERTIA INCREASES (INERTIA = MASS)

Part C: Newton’s Laws of Motion VI. Newton’s First Law of Motion Newton’s 1st Law of Motion: Which of the following problems have an unbalanced force (net force does not = 0)? a. A car is traveling at 20 meters per second, and the driver steps on the brake. b. A chair is dragged across the floor at a constant velocity. c. A student is sitting still in a chair. d. A plane makes a turn at a constant speed. e. An object is dropped vertically toward the earth. AN OBJECT WILL REMAIN AT REST OR AT CONSTANT VELOCITY UNTIL AN UNBALANCED FORCE ACTS ON IT

Seatbelt Law Video (2 min) Part C: Newton’s Laws of Motion XII. Newton’s First Law of Motion Newton’s 1st Law of Motion and the seatbelt Seatbelt Law Video (2 min) Table Cloth Stunt from Mythbusters Part 2 - 1 min

Part C: Newton’s Laws of Motion VII. Newton’s Second Law of Motion Newton’s 2nd Law of Motion: Equation: (see reference tables) ACCELERATION IS DIRECTLY PROPORTIONAL TO THE FORCE AND INVERSELEY PROPORTIONAL TO MASS

Part C: Newton’s Laws of Motion VII. Newton’s Second Law of Motion Examples: 1. What net force is required to accelerate a 1500 kg truck at 5 m/s2?

Part C: Newton’s Laws of Motion VII. Newton’s Second Law of Motion Examples: 2. A. What is the net horizontal force? B. What is the acceleration (magnitude and direction) of the block below?

Part C: Newton’s Laws of Motion VII. Newton’s Third Law of Motion Newton’s 3rd Law of Motion: Action/Reaction Pairs: David Ortiz hits a baseball with a force of 200 N. How much force does the baseball exert on the bat? FOR EVERY ACTION (FORCE) THERE IS AN EQUAL AND OPPOSITE REACTION (FORCE) 200 N

Part C: Newton’s Laws of Motion VII. Newton’s Third Law of Motion What are “action” and “reaction” “action” and “reaction” are names of _______________ Forces __________ come in ___________ FORCES ALWAYS PAIRS

Part C: Newton’s Laws of Motion VII. Newton’s Third Law of Motion Action/Reaction Forces Since a force is an interaction between _________ objects, ____________ objects are involved in ______________ Action force: Reaction force: TWO TWO EVERY FORCE A PUSHES B B PUSHES A

Part C: Newton’s Laws of Motion VII. Newton’s Third Law of Motion D. “Equal” In Newton’s 3rd Law, “equal” means equal in ______________ Action and reaction forces are exactly the same _________ Action and reaction forces occur at exactly the same ________ SIZE AND TIME SIZE (MAGNITUDE) TIME

a m Fnet Fnet = Part C: Newton’s Laws of Motion = DIRECTION SAME FORCE VII. Newton’s Third Law of Motion E. “Opposite” In Newton’s 3rd Law, “opposite” means ______________ F. Don’t Forget about Mass The __________________ acting on objects of ______________ will produce _______________________ DIRECTION Same force Fnet Fnet SAME FORCE DIFFERENT MASSES a = = m a m DIFFERENT ACCELERATIONS

a m Fnet Fnet = Part C: Newton’s Laws of Motion = VII. Newton’s Third Law of Motion Don’t Forget about Mass Example A bug flies into a moving truck. How does the force exerted on the truck compare to the force exerted on the bug? Same force Fnet Fnet a = = m a m THE SAME

Part C: Newton’s Laws of Motion Friction Normal Force: Frictional Force: Equation (See reference tables): Force from surface pushing on object (always perpendicular to surface) Force that opposes motion (always acts parallel to surface)

Part C: Newton’s Laws of Motion Friction Static Friction: If you pull on an object and it does not move, how does your applied force compare to the force of static friction? FORCE OF FRICTION THAT PREVENTS OBJECTS TO START MOTION EQUAL, BUT OPPOSITE DIRECTION (NOTE: this is not Newton’s 3rd Law)

Part C: Newton’s Laws of Motion Friction Static Friction: Maximum static friction (Ff, static max) occurs: 0 Ff, static Ff, static max OBJECT JUST BEGINS TO MOVE

Part C: Newton’s Laws of Motion Friction Static Friction: Example: What is the minimum horizontal force a man has to pull on a 200 N wood crate on a wood floor to start it to move? (use reference tables) Phone Book Friction Mythbusters (6 min video)

Part C: Newton’s Laws of Motion Friction Kinetic Friction: Physics of Curling (4 min) How ABS Work (1 min) Ff, kinetic Ff, static Equation: FORCE OF FRICTION THAT OPPOSES MOTION OF A MOVING OBJECT

Part C: Newton’s Laws of Motion Kinetic Friction: Example: Jim exerts a 36 N horizontal force as he pulls a 5 kg sled across a cement sidewalk at a constant speed. What is the coefficient of kinetic friction between the sidewalk and the metal sled?

Applications of Friction Friction Welding Ice Storm in Portland ABS Brakes (Kinetic vs. Static Friction) (video) Physics of Curling (start at 2:30 video) Sled wax (Christmas Vacation)

Part C: Newton’s Laws of Motion Practice Problems Complete the following problems on a whiteboard and jot down the correct answer once checked Newton’s 2nd Law: 2,000 kg 2.4 kg 100 m/s A) 2 N B) 1 m/s2

Part C: Newton’s Laws of Motion Practice Problems Complete the following problems on a whiteboard and jot down the correct answer once checked Friction: 58.9 N A) drawing B) 0.66 A) drawing, B) 98.1 N, C) 98.1 N, D) Zero, not moving vertically, E) 19.62 N, F) 38.38 N, G) 3.04 m/s/s A) 2 N B) 1 m/s2