Unit 4: Forces and Newton’s Laws of Motion

Unit 4: Forces and Newton’s Laws of Motion
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Unit 4: Forces and Newton’s Laws of Motion
Essential Questions/Skills: What is a resultant and equilibrant? What does it mean for an object to be in equilibrium? How can you find the resultant and equilibrant algebraically and graphically? What is the difference between mass and weight? How do you find weight? Calculate the normal force and force of friction for an object on an incline plane What are Newton’s 3 Laws of Motion? Perform calculations using Newton’s 2nd Law of Motion. What is the difference between static and kinetic friction? Know how to calculate both of them.

Part A: Statics and Vectors
AT REST I. Statics: study of objects ________________ OR at __________________ (acceleration equals ________) II. Force What is it? What does it do? Units: CONSTANT VELOCITY ZERO PUSH OR PULL CAUSES AN OBJECT TO ACCELERATE NEWTONS (N) or kg*m/s2

III. Sum of Forces Acting on an Object When multiple forces act on a single object at the same point we say the forces are acting ________________ on the object. Resultant: Equilibrium: Equilibrant: CONCURRENTLY SUM of the vectors (we will focus on forces as the vectors for this unit) When the SUM of the forces equal ZERO (acceleration = ZERO , so it is at REST or CONSTANT VELOCITY) Force that brings equilibrium state. (Same MAGNITUDE, but OPPOSITE direction of resultant)

Algebraic Method of Finding Resultant/Equilibrant Example: A 10 N force at 0 degrees acts concurrently with a force of 15 N at 180 degrees on an object. A) Calculate the resultant of the forces. B) Calculate the equilibrant of the forces.

Algebraic Method of Finding Resultant/Equilibrant Example: A 20.0 N force at 90.0 degrees acts concurrently with a force of 30.0 N at 0.00 degrees. A) Calculate the resultant of the forces. B) Calculate the equilibrant of the forces.

Graphical Method of Finding Resultant/Equilibrant Use _______________ method to find the resultant/equilibrant. Example: A 10 N force at 0 degrees acts concurrently with a force of 15 N at 180 degrees on an object. Using a scale diagram find the: A) the resultant of the forces. B) the equilibrant of the forces. HEAD TO TAIL

Graphical Method of Finding Resultant/Equilibrant Example: A 20 N force at 90 degrees acts concurrently with a force of 30 N at 0 degrees. Using a scale diagram find the: A) the resultant of the forces. B) the equilibrant of the forces. degrees

Graphical Method of Finding Resultant/Equilibrant Example: A 20 N force at 0 degrees acts concurrently with a force of 15 N at 70 degrees on an object. Using a scale diagram find the: A) the resultant of the forces. B) the equilibrant of the forces.

Graphical Method of Finding Resultant/Equilibrant Example: A 80 N force at 0 degrees acts concurrently with a force of 100 N at 65 degrees. Using a scale diagram find the: A) the resultant of the forces. B) the equilibrant of the forces.

Part B: Weight vs. Mass Forces cause acceleration Force Equation
Equation (see Reference Tables): Example: What net force is required to accelerate a 1500 kg race car at + 3 m/s2? Forces cause acceleration

Part B: Weight vs. Mass Mass vs. Weight 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. Mass is a CONSTANT measurement Weight is known as the FORCE DUE TO GRAVITY a = g = 9.81 m/s2 (ON EARTH)

Part B: Weight vs. Mass 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 the object Always acts PERPENDICULAR to the surface

Part B: Weight vs. Mass Apparent Weight
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 (at rest or constant velocity)

Part B: Weight vs. Mass 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)

Part B: Weight vs. Mass 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

Example: The elevator from the above example begins to move. B) What would happen to the scale reading if the elevator accelerates down?

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 Aboard the Vomit Comet video (4 min)
Change in Weight Aboard the Vomit Comet video (4 min)

Part B: Weight vs. Mass XI. Object on an Incline Plane
A. Looking at the Angles on a Right Triangle

Components of Weight on an Incline The box has a mass of 20 kg and rests on an incline angled at 50 degrees.

Components of Weight on an Incline What is the weight (force due to gravity) of the box? What are the x and y components of Fg?

Components of Weight on an Incline What are the magnitudes of the normal and force of friction? A) Normal Force: B) Force of Friction:

Part B: Weight vs. Mass Practice Problem
A 10 kg turkey remains at rest on a 40 degree incline. A) Draw all forces acting on the turkey

Part B: Weight vs. Mass Practice Problem Fg = _________ Fgx = ________
A 10 kg turkey remains at rest on a 40 degree incline. B) Calculate the force due to gravity, the horizontal and vertical components of the force of gravity, normal force, and force of friction? Fg = _________ Fgx = ________ Fgy = ________ FN = ________ Ff = _________

Part C: Newton’s Laws of Motion
XII. Newton’s First Law of Motion Inertia: Units: Examples: 1. Which object has more inertia: 2 kg pumpkin moving at m/s or 10 kg turkey at rest? 2. What happens to the inertia of an object, if the mass increases? Tendency for an object to remain at rest or constant velocity (a = 0) kg 10 kg TURKEY (MORE MASS) INERTIA INCREASES (INERTIA = MASS)

XII. 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

XII. Newton’s First Law of Motion Newton’s 1st Law of Motion and the seatbelt Classic Table Cloth Trick (video) Table Cloth Stunt from Mythbusters (video)

XIII. Newton’s Second Law of Motion Newton’s 2nd Law of Motion: Equation: (see reference tables) Practice Drawing Free Body Diagrams on select pages in the Work Packet Acceleration is DIRECTLY proportional to the net force and INVERSELEY proportional to the mass

XIII. Newton’s Second Law of Motion Examples: 1. What net force is required to accelerate a 1500 kg truck at 5.00 m/s2?

XIII. 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?

XIII. Newton’s Second Law of Motion Examples: A 0.3 kg toy car starts from rest and reaches a final velocity of 10 m/s after 5 seconds. What was the amount of force exerted on it?

XIV. Newton’s Third Law of Motion Newton’s 3rd Law of Motion: Action/Reaction Pairs: For every ACTION (FORCE) there is an EQUAL and OPPOSITE REACTION (FORCE)

XIV. Newton’s Third Law of Motion David Ortiz hits a baseball with a force of 30,000 N. How much force does the baseball exert on the bat? 30,000 N Falcon 9 Rocket (SpaceX)

XV. 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)

XV. Friction Static Friction: (phone book 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 prevent objects to START motion EQUAL, BUT OPPOSITE DIRECTION (NOTE: this is not Newton’s 3rd Law)

XV. Friction Static Friction: Maximum static friction (Ff, static max) occurs: Ff, static Ff, static max OBJECT JUST BEGINS TO MOVE

XV. 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)

XV. Friction Kinetic Friction: Ff, kinetic Ff, static Equation: Force of friction that opposes motion of a MOVING object

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)

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?

Friction Static Friction: WHITEBOARD PROBLEM Example: A person pushes on a 800 kg car with a force of 6,500 N and the car just begins to move. Sketch a free body diagram for the system. What is the force of friction between the tires and the ground? What is the coefficient of friction between the tires and the ground? 6,500 N 0.828

Friction Kinetic Friction: WHITEBOARD PROBLEM Example: Two people push on a 800. kg car with a combined force of 6,500 N to have it accelerate at a constant rate. Draw a free-body diagram of forces on the car. Calculate the amount of friction there is between the rubber tires and dry asphalt. Calculate the net horizontal force. Calculate the acceleration of the car.. B) 5,258 N, C) 1240 N, D) 1.55 m/s/s

XVI. Practice Problems Complete the following problems on a whiteboard and jot down the correct answer once checked Newton’s 2nd Law: (from pg 2,000 kg 2.4 kg 100 m/s A) 2 N B) 1 m/s2

XVI. Practice Problems Complete the following problems on a whiteboard and jot down the correct answer once checked Friction: (pg. 14) 58.9 N A) see below B) 0.66 A) see below, B) 98.1 N, C) 98.1 N, D) Zero, not moving vertically, E) N, F) N, G) 3.04 m/s/s 222 N

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