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Forces and Circular motion

Forces and Circular motion. I. Force. A. Definition: a push or pull acting on a mass 1. Force is a vector quantity with both magnitude (numeric value) and direction 2. Force can be broken down into horizontal and vertical components 3. Symbol: 4 . Units: . F. Newtons ! (N).

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Forces and Circular motion

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  1. Forces and Circular motion

  2. I. Force A. Definition: a push or pull acting on a mass 1. Force is a vector quantity with both magnitude (numeric value) and direction 2. Force can be broken down into horizontal and vertical components 3. Symbol: 4. Units: F Newtons! (N)

  3. B. Concurrent Forces: many forces acting on the same object at the same time. 1. Treat all forces individually to find a resultant force (break into components) 2. This resultant of all concurrent forces is called the Net Force Fnet Symbol:

  4. C. Free Body Diagram: represents concurrent forces acting on an object

  5. Example 1: Net force • Two Physics students try pushing a car to see who is stronger. One student pushes west with a force of 500 Newtons. The other pushes East with a force of 700 Newtons. • Draw a free body diagram of the situation. • What is the Net Force? • What way does the car move?

  6. Example 2: net force • Two Physics students are again arguing and this time are in a tug of war. They are pulling on a box. One student pulls 30 degrees toward the northeast with a force of 400 Newtons and the other pulls at 20 degrees toward the Northwest with 500 Newtons. • Draw a free body diagram of the situation. • What is the Net Force?

  7. Journal #11 10/1 • What is net force on a box of mice being pulled with a force of 20 Newtons due West toward a snake pit and another force of 30 Newtons pulling due East toward an alley filled with cats, a 50 Newton force pulling due North toward a cliff, and a 50 Newton force pulling due South toward a large pond? • Draw a Free Body Diagram 1st!!!

  8. D. Static Equilibrium: reached when the resultant of all forces acting on an object is ZERO (balanced) 1. At Equilibrium, objects remain at rest or constant velocity. Fnet= Zero Fnet≠ Zero

  9. 2. Net Force is equal to ZERO in static situations Fnet=0

  10. Example: Static Equilibrium • What forces MUST be added in order to produce static equilibrium in the free body diagram below?

  11. II. Dynamics Effects of forces acting on objects (Newtons Laws of Motion) A. Newton’s First Law: An object maintains a state of equilibrium unless acted on by an unbalanced force. (at rest or constant velocity) 1. Any unbalanced force (Fnet≠ 0) will produce a change in an object’s velocity…either speed, direction, or both. •the object will ACCELERATE 2. Newton’s First Law is also known as the Law of Inertia •Inertia: the resistance of an object to a change in its motion  More Mass = More Inertia

  12. • Masses resist changes in motion…

  13. Examples: Inertia • What has more inertia? A 10 kg bag of feathers sitting still or a 5 kg gold bar moving along at 10 m/s? • What has more inertia? A 20 kg baseball sitting on a stand, or a 5 kg bowling ball moving along at 30 m/s?

  14. B. Newton’s Second Law: the acceleration of an object is directly proportional to the net external force acting on an object and inversely proportional to the object’s mass. •force is related to mass and acceleration using the famous expression: •acceleration is produced by force(s) • increasing force will increase the acceleration

  15. 1. Units for Force…Yay!! Dimensional Analysis! a. Newtons are the SI unit of force and are a derived unit (combination of fundamental units) b. 1 Newton is equal to the force required to accelerate a 1 kilogram mass 1 meter per second squared

  16. 2. Increasing mass will increase the force needed to accelerate that mass larger m larger Fnet *The equation must balance!

  17. 3. If the force is constant, then increasing the mass of an object will decrease the resulting acceleration Fnet  a  m   a

  18. 4. Graphing Fnet = ma : Direct Relationship: Increasing Force produces more acceleration Acceleration (m/s2) Force (N)

  19. Example: Newton’s 2nd Law A capybara with a mass of 100kg is tackled by a Jaguar with a steady force of 100 N along the ground. Assuming no friction, what is the acceleration of the rodent?

  20. 5. On HORIZONTAL surfaces… only the HORIZONTAL component of a force will accelerate an object.

  21. C. Newton’s Third Law: when one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude, but opposite in direction.  For every action there is an equal and opposite reaction!

  22. • What happens to a boat when you step onto a dock? Newton’s 3rd Law!!!

  23.  Newton’s 3rd Law also applies in space when making objects move

  24. III. Natural Forces A. Weight: gravitational force exerted on a small mass by a planet/large body 1. Weight CHANGES based on what planet/object you are on… MASS does NOT CHANGE 2. Symbol: 3. Units: 4. Equation: Newtons! (N) How much do you weigh?

  25. Example: Weight • The fattest, ugliest Capybara has a mass of 66 kg. What is the weight of the rodent on Earth? • Convert the mass to pounds if 1 kilogram = 2.2 pounds

  26. B. Newton’s Universal Law of Gravitation: Describes the force of attraction between different masses.  Any two bodies attract each other with a force that is directly proportional to the product of their masses, and inversely proportional to the square of the distance between them

  27. Fg= Gravitational Force G= Universal Gravitational Constant = 6.67 x 10-11 N•m2/kg2 m1= mass of object 1 m2= mass of object 2 r= distance between the two masses On Your Reference Tables!! (Front Cover)

  28. • Graphical Representation:

  29. Example: Newton’s Universal Law of gravitation What is the force of gravitational attraction between the Earth and the Moon? m1 = Earth = 5.98 x 1024 kg m2 = Moon = 7.35 x 1022 kg r = 3.84 x 108 m G= 6.77 x 10-11 N•m2/kg2

  30. Journal #12 10/7 A 200 kg box of rodents is sitting on the road. A truck pulls the crate with a force of 300 Newtons to the East while another truck pulls the crate with a force of 150 Newtons to the West. • Draw a free body diagram and label all forces… • What is the net force on the crate? • Disregarding friction, what will the acceleration of the crate be? • Calculate the weight of the crate of the rodents? • What is the force of gravitational attraction between the crate of rodents and the Moon?

  31. 2. Gravitational Fields: vectors are used to show gravitational force  A “unit test mass” will accelerate along gravitational field lines, toward the center of the source of gravity

  32. C. The Normal Force: force exerted on an object perpendicular to the surface of contact a. Prevents objects from accelerating due to gravity b. Usually denotedFNFnor N

  33. c. For horizontal surfaces, the normal force perfectly balances/cancels Fg (weight) 90°

  34. d. If the object is on an incline, the normal force is angled from vertical the same amount as the incline θ 90° θ

  35. e. The components of the Normal force f. Component of Fgthat is perpendicular to the surface always balances FN

  36. g. If the object “slides” down the surface, the force responsible is the component of gravity acting along the ramp (if it doesn’t slide, it must haveanother force balancing gravity!)

  37. D. Applied Force: the actual direction of the push or pull • an object may accelerate in the direction of the applied force if it is strong enough

  38. E. Force of Friction: opposes applied force or relative motion of two objects in contact with each other Symbol: Ff a. If applied force exceeds frictional force, the object will accelerate in the direction of the applied force

  39. b. Factors Affecting Force of Friction 1.Compression: Increasing the force pressing objects together increases force of friction  on Earth, weight and normal force usually compress objects friction is directly proportional to the normal force and weight  BEWARE: normal force and weight are only equal on horizontal, flat surfaces!

  40. 2. Surface Area of contact between surfaces of materials has NO effect on force of friction because an object’s weight is distributed over the surface of contact regardless of size

  41. 3. Type of Surface Roughness = Friction 4.Speed vs. Stationary: Moving objects experience less resistance while continuing movement than to start moving

  42. Journal #14 10/9 A 20 kg crate is sitting on a 35 degree incline. There is friction between the ramp and the crate. • Draw a free body diagram and label all forces… • Calculate the weight of the crate • What is is the normal force of the incline on the crate? (Break apart Gravity!)

  43. c. Static Friction: opposes applied force on objects at rest and resists motion d. Kinetic Friction: opposes motion of objects in contact that are already moving e. Kinetic friction is LESS than Static friction due to action of molecular forces

  44. f. Coefficient of Friction: ratio of frictional force to normal force – denoted μ  so the force of friction is equal to:  essentially, the force of friction is a fraction of the normal force

  45. Example: Friction! • A large wooden crate of (you fill in the blank) is sitting on a level wooden plank. The mass of the crate is 100 kg. A super strong physics teacher pushes on the crate with a force of 500 Newtons East. FRICTION opposes that force. • Sketch a free body diagram of the situation showing ALL forces acting on the crate. • What is the Weight of the crate? • What is the Normal Force on the crate from the roadway? • What is the force of Static Friction on the crate? • Why does the crate move? • What is the new Force of Friction on the moving crate? • What is the Net Force on the moving crate? • What is the acceleration of the crate?

  46. 1. Coefficients of friction are listed in the Reference Tables!! Reference Tables – Front Cover!!

  47. 2. Since the force of static friction is greater than the force of kinetic friction, the coefficients change as well

  48. 3. Equations become: • treat static and kinetic situations separately

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