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Chapter 12

Chapter 12. Forces & Motion. Objectives. Describe examples of force and identify appropriate SI units Explain how the motion of an object is affected when balanced and unbalanced forces act on it Compare and contrast the four kinds of friction

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Chapter 12

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  1. Chapter 12 Forces & Motion

  2. Objectives • Describe examples of force and identify appropriate SI units • Explain how the motion of an object is affected when balanced and unbalanced forces act on it • Compare and contrast the four kinds of friction • Describe how Earth’s gravity and air resistance affect falling objects

  3. Force • Definition - _______ or _______ that acts on an object - can accelerate or decelerate an object ex. • Measured - units: _________ (N) = 1 kg m/s2 • Represented - force vectors: arrow; direction length; _________ or __________

  4. Forces Cont. • Combining Forces - forces acting in the same directions: ______ - force in opposite directions: _________ • Net Force - the __________ force acting on an object after all the forces are combined • Two Types - ___________ - unbalanced

  5. Balance vs. Unbalanced Forces • Balanced forces - forces that combine to produce no net force ex. - no net force, no _____________ • Unbalanced forces - force that results when the net force acting on an object is not ________ to 0 ex. - object will move in direction of _____ ________

  6. Practice Problems • You push a car with a force of 50 N, you friends pulls with a force of 25 N. Draw force diagrams, and calculate the net force acting upon the car. Will the car move? • You push a box towards your friends with a force of 80N while one friend pushes the box against you with a 55 N, the other 25N. Draw force diagrams and calculate the the net force acting upon the car. Will the box move?

  7. Common Forces • Two common forces - _________: force that acts between any two masses - attractive force that can act over a large distance - Earth’s gravity acts downward toward the center of Earth = ______________________ ex:

  8. Common Forces Cont. • Friction - force that _________ the motion of objects that ________ as they move past one another - ______ types: static, sliding, rolling, fluid - static: acts on objects that are not ______ ex. - _______: force that opposes the direction of motion of an object as it slides over a surface ex.

  9. Common Forces Cont. - rolling friction: force that acts on ________ objects ex. - fluid friction: force that _________ the motion of an object through _________ * any mixture of gases is considered a fluid ex.

  10. Force Diagrams • A diagram that identifies all forces acting upon on object 1. Identify the __________ of the object ex. 2. Identify the _________ acting upon the object ex. 3. Draw the forces acting upon the object ex. forces cancel out • net force = ____

  11. Practice Problems • An object is decelerating due to friction • An object is static • An object hanging from the ceiling

  12. Objectives • Compare and contrast Aristotle Galileo’s and Newton’s ideas • Define inertia • Explain Newton’s 1st law of motion, and apply them to physical situations

  13. Galileo Vs. Newton: Inertia • Aristotle - believed that natural laws could be explained by logical reasoning ex. heavier object fall faster than lighter moving objects must have a forces applied to keep moving • Galileo - introduced the idea of inertia - maintained that the ___ of an object requires a _________ • Newton - grasped the significance of inertia - motion of an object requires an initial force, not continual. - law of inertia defines natural motion and tells us what kinds of motion are the result of applied forces

  14. Newton’s First Law of Motion - Newton redefined Galileo’s idea of inertia Inertia: tendency of things to _______ a change in it’s _________ • Law 1 - Every material object ____________ in its state of rest or uniform motion in a __________ ______, unless it is compelled to change that state by ________ impressed upon it - Key Word:

  15. Newton’s 1st Law Cont. ex. * Holds true whether your are at rest or moving at a constant velocity ex.

  16. Inertia Problems Q: A hockey puck sliding across the ice finally comes to rest. How would Aristotle interpret this behavior? How would Galileo and Newton interpret it? A: Aristotle: Galileo & Newton:

  17. Objectives • Be able to understand and apply Newton’s 2nd Law of motion • Using Newton’s 2nd: calculate force, mass, and acceleration • Recognize that the free fall acceleration near Earth’s surface is independent of the mass of the falling object • Explain the difference between mass and weight

  18. Newton’s 2nd Law • The acceleration of an object is ___________ proportional to the _____ _______ acting on the object, in the direction of the net force, and is __________ proportional to the ______ of the object - in short: - symbols: a = acceleration Fnet = force m = mass

  19. Practice Problems • A boy pushed forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? • An automobile with a mass of 1200 kg accelerates at a rate of 3.0m/ s2 in the forward direction. What is the net force acting on the automobile? • A 25 N force accelerates a boy in a wheelchair at 0.5m/s2. What is the mass of the boy and the wheelchair?

  20. Acceleration • Acceleration can be: • Equal to 0: Net Forces = _____ ___________ motion - object is not ________ or ____________ - all forces are ___________ ex. _________ motion - object is __________ but not ___________ - all forces are _____________ ex.

  21. Acceleration Cont. • < gravity: net forces >0 - object is moving & experiencing _______ - forces are _________ ex. *important to remember NET FORCES ex. air resistance is neglected: net force is the ___________ ____________ ex. presences of air resistance: net force is _____ than the weight objects weight – _______________

  22. Acceleration Cont. • Air resistance of an object depends on 2 factors 1. The _________ area (surface area) of an object - greater the area greater the air resistance 2. The _________ of the object - greater the speed greater the air resistance

  23. Acceleration Cont. • Terminal Speed - When acceleration of an object equals zero • If concerned with direction; use terminal velocity • Why? - Velocity indicates a ___________ and _______

  24. Acceleration Cont. Ex. Skydiving - as you fall you gain speed, air resistance therefore builds until finally it equals your weight. If this happens, the net force is equal to zero and you no longer accelerate, reaching terminal speed feather few ~ centimeters per second sky diver ~ 200 kilometers per hour

  25. Acceleration Cont. • = gravity (free fall) - object is falling at _________ - air resistance can be ___________ ex. Why does the object with double the mass not accelerate greater?

  26. Acceleration Problem Q: A jumbo jet cruises at a constant velocity of 1000 km/hr when the thrusting force of it’s engine is a constant 100,000 N. What is the acceleration of the jet? What is the force of air friction (air resistance) on the jet? A:

  27. Air Drag Problems Q: Consider a man and a woman parachuting together from the same altitude. Suppose the man is twice as heavy as the woman and that their same sized chutes initially open simultaneously. Who gets to the ground first? A:

  28. Air Drag Problems Cont. Q: A skydiver jumps from a high flying helicopter. As she falls faster and faster through the air, does her acceleration increase, decrease, or remain the same? A:

  29. Objectives • Explain Newton’s 3rd law of motion and relate it to everyday events • Explain how action and reaction forces are related according to Newton’s 3rd law • Be able to define a system of interactions

  30. Newton’s 3rd Law • Action Reaction Forces - whenever on object exerts a force on a second object, the second object exerts an ______ and _________ force on the first object. ex. produces motion: car tires push push _________ against the road the road pushes ________ against the car tires ex. no motion: hand ________ against the wall, wall ________ against your hand

  31. Newton’s 3rd Law Cont. • if we drop a sheet of tissue paper in front on the heavy weight champion of the world and challenge him to hit it in midair with 50 lbs of force, his best punch couldn’t even come close Why? - a force is not a thing in itself but makes up an interaction between itself and another (________) - the _______ force and the _________ force - net force only equals 0 when force are ________ and _________ ex.

  32. System of Interactions • Define the system - Jack and Jill: Nf =0, hence no acceleration, force is _________ to the system ex. - Jack: Nf > 0, acceleration, force is _________ to the system ex. - Jill: = Nf > 0, acceleration, force is _________ to the system ex.

  33. Systems of Interactions Problems Q: One cold and rainy day, you car battery is dead and you must push the car to get it started. Why can’t you push the car by remaining comfortably inside pushing against the dash? A:

  34. Objectives • Define momentum • Understand how momentum and Newton’s laws related • Explain how impulses affect momentum

  35. Momentum • Definition • a quantity defined as the ________ of an object’s ________ and its ________ • Formula - • Units • mass = • velocity = • therefore

  36. Momentum Problems • Which object has more momentum: a car traveling at a speed of 10k m/hr or a baseball pitched at 150 km/hr. Explain your answer • What is the momentum of an 0.30 kg blue jay flying at 17 m/s? • What is the mass of the train car moving at 14 m/s with a momentum of 140 kg-m/s? of the car moving at 10 m/s with a momentum of 100kg-m/s? What is the total momentum of the system?

  37. Momentum Cont. • Why is a heavy truck harder to stop than a small moving car at the same speed? - • Can you change an objects momentum? - , using forces, but most importantly “____ ______” that force is applied ex. ex.

  38. Impulses • Definition - product of _______ and this _______ interval • General - the relationship between impulse and momentum can be seen by rearranging Newton’s 2nd law (a=F/m) - time interval part of impulse is “buried” in the term for acceleration (change in v/t interval) - F x t interval = change in (mass x velocity) -shorthand:

  39. Impulses Cont. • rearrangement of Newton’s 2 law explains, why “follow through” is important in increasing the momentum of things Q: Would there be a difference in the momentum of a long barrel cannon or a short barrel cannon, and if so which would be greater? A:

  40. Impulses Cont. • decrease momentum over a long time, a ________ force results ex. *If you wish the force of impact to be smaller, extend to time of impact Boxer:

  41. Momentum Problems Q: Explain how a karate expert can sever a stack of bricks with the blow of his bare hands. A: ***Remember small “t” large “F”

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