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Bellwork

Bellwork. Pick up a free-body diagram sheet and begin working on it. October 21, 2009. SWBAT state and calculate weight SWBAT solve for coefficients of static and kinetic friction Catalyst Hang on to worksheets from yesterday. Weight & Friction. Weight & Friction.

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Bellwork

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  1. Bellwork • Pick up a free-body diagram sheet and begin working on it.

  2. October 21, 2009 • SWBAT state and calculate weight • SWBAT solve for coefficients of static and kinetic friction Catalyst • Hang on to worksheets from yesterday

  3. Weight & Friction

  4. Weight & Friction Weight – the mass and acceleration of an object due to gravity

  5. Weight & Friction Weight – the mass and acceleration of an object due to gravity creates weight.  aka - Fg

  6. Weight & Friction Fg = mg Force of Gravity = mass x gravity

  7. Weight & Friction Fg = mg An object with a mass of 9kg has what weight or Fg?

  8. Weight & Friction Fg = mg An object with a mass of 9kg has what weight or Fg? Fg = (9)(9.81)

  9. Weight & Friction Fg = mg An object with a mass of 9kg has what weight or Fg? Fg = 88.3N

  10. Weight & Friction What force is resisting the box?

  11. Weight & Friction What force is resisting the box?

  12. Weight & Friction This is called the Normal Force (Fn)

  13. Weight & Friction Normal Force (Fn) – The upward force equal to the downward Fg exerted by a surface.

  14. Moving a Refrigerator • Draw a free body diagram for the following situation. • A man pushes against a 85 kg refrigerator with 50 N of force but it doesn’t go anywhere.

  15. Weight & Friction • Friction (Ff) – The force that opposes the relative motion of two surfaces in contact • Friction is proportional to the Fn and depends on the surfaces in contact.

  16. Weight & Friction • Static friction (Fs) = the resistive force that keeps something from moving when at rest • Static means not moving • Fs = -Fapplied • Fs = μsFn where μs is the coefficient of static friction (it has no units)

  17. Moving the Fridge • Draw a free body diagram for the following situation. • A man pushes against a 85 kg refrigerator with 50 N of force but it doesn’t go anywhere • What is the coefficient of static friction between the floor and the fridge?

  18. Kinetic Friction • Objects also experience friction while they are moving • For moving object, friction: • Points opposite the direction of motion • Is parallel to the surface • The equation is Fk=μkFn where μk is the coefficient of kinetic friction (also no units)

  19. Weight & Friction • Question. Think about pushing a box full of books across the floor. Is it harder to push before or after it starts moving? • So would you expect the coefficient of static or kinetic friction to be a greater value? (indicating more Friction)

  20. Weight & Friction • A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. • Figure out a plan • Try to solve without putting the numbers in first. • µs = .32

  21. Weight & Friction • a 91kg refrigerator is placed on a ramp. The refrigerator begins to slide when the ramp is raised to an angle of 34˚. What is the coefficient of static friction • Figure out a plan • Try to solve without putting the numbers in first. • µ = .67

  22. Weight & Friction • A student moves a box of books by attaching a rope to the box and pulling with a force of 90.0N at an angle of 30.0˚. The box of books has a mass of 20.0kg, and a coefficient of kinetic friction between the bottom of the box and the sidewalk is .50. Find the acceleration of the box. • Steps (how are we going to find our answer?) • Draw diagram • Find Fn using ( ΣFy = 0) • Calculate the force of kinetic friction (Fk = µkFn) • Use Newton’s 2nd Law on the horizontal axis to find acceleration (ΣFx = max) • a = .12m/s2 to the right

  23. Group Practice Problem! • Two students are sliding a 225kg sofa at a constant speed across a wood floor. One student pulls with a force of 225N at an angle of 13° above the horizontal. The other student pushes with a force of 250N at an angle of 23° below the horizontal. What is the coefficient of kinetic friction between the sofa and the floor? • Steps (how are we going to find our answer?) • Draw diagram • Find Fk using ( ΣFy = 0) • Calculate the coefficient of kinetic friction (Fk = µkFn) • μk = .20

  24. Pg 145; #1, 2, 3a, 3b Pg 147; #1, 3, 4 Practice Work

  25. Weight & Friction A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor

  26. Weight & Friction Ff = μFn A block sliding (μ = .57) on a surface has a normal force of 13N, what is the friction force? Ff = (.57)(13)

  27. Weight & Friction Ff = μFn A block sliding (μ = .57) on a surface has a normal force of 13N, what is the friction force? Ff = 7.4N

  28. Weight & Friction Ff = μFn A block sliding (μ = .1) on a surface has a friction force of 8N, what is the normal force?

  29. Weight & Friction Ff = μFn A block sliding (μ = .1) on a surface has a friction force of 8N, what is the normal force? 8 = (.1)(Fn)

  30. Weight & Friction Ff = μFn A block sliding (μ = .1) on a surface has a friction force of 8N, what is the normal force? Fn = 80N

  31. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force?

  32. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force?

  33. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force? Fg = mg

  34. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force? Fg = 30(9.81)

  35. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force? Fg = 294.3N

  36. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force? Fg = 294.3N Fg = Fn

  37. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force? Fn = 294.3N

  38. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force? Fn = 294.3N Ff = (.2)(294.3)

  39. Weight & Friction Ff = μFn A 30 kg block sliding (μ = .2) on a surface has what friction force? Fn = 294.3N Ff = 58.9N

  40. Weight & Friction What net force is being on the box below?

  41. Weight & Friction What net force is being on the box below? 222N

  42. Weight & Friction If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force?

  43. Weight & Friction Fg = mg If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force? Fg = 33(9.81)

  44. Weight & Friction Fg = mg If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force? Fg = 323.7N

  45. Weight & Friction Fg = mg If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force? Fg = 323.7N = Fn

  46. Weight & Friction Ff = μFn If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force? Ff = (.03)(323.7)

  47. Weight & Friction Ff = μFn If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force? Ff = 9.7N

  48. Weight & Friction Ff = μFn If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force? Fnet = 22 - 9.7

  49. Weight & Friction Ff = μFn If a 22N force is being applied to a 33kg block sliding (μ = .03) on a surface, what is the net force? Fnet = 22 - 9.7 = 12.3

  50. Weight & Friction Fnet = ma Take it a step further, how fast is the block accelerating?

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