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Video Link: Cars sliding down a snowy Seattle hill: crashing steel Ice Capades - Boing Boing

Video Link: Cars sliding down a snowy Seattle hill: crashing steel Ice Capades - Boing Boing. What is ATM? Anti traction material What is MDS? Mobility denial system. What is a force? A force is any influence that can change the velocity of a body.

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Video Link: Cars sliding down a snowy Seattle hill: crashing steel Ice Capades - Boing Boing

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  1. Video Link: • Cars sliding down a snowy Seattle hill: crashing steel Ice Capades - Boing Boing

  2. What is ATM? • Anti traction material • What is MDS? • Mobility denial system

  3. What is a force? A force is any influence that can change the velocity of a body. Forces can act either through the physical contact of two objects (contact forces: push or pull) or at a distance (field forces).

  4. How do we measure forces? Spring Scales are used to measure forces. The spring inside the scale stretches an amount proportional to the force applied and can be calibrated (scaled)

  5. WEIGHT The weightof a body is the gravitational force with which the Earth attracts the body. Weight (a vector quantity) is different from mass (a scalar quantity). The weight of a body varies with its location near the Earth (or other astronomical body), whereas its mass is the same everywhere in the universe. The weight of a body is the force that causes it to be accelerated downward with the acceleration of gravity g. Weight = mass x gravity W = m x g Newton = kg x m/s2 1 N = 1 kgm/s2

  6. The normal force is a force that keeps one object from penetrating into another object. The normal force is always perpendicular a surface. The normal exactly cancels out the components of all applied forces that are perpendicular to a surface.

  7. What units are used for FORCES? In Metric… Newton (N) is the amount of force required to give a 1-kg mass an acceleration of 1 m/s2. In English… is the amount of force required to give a 1-slug mass an acceleration of 1 ft/s2. Pound (lb)

  8. Conversions or Equivalents 1 pound = 4.45 Newtons 1 kilogram weighs 9.80 N = 2.2 lbs

  9. How do we describe forces? Forces have a magnitude & a direction

  10. Free-body Diagrams Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. • The size of the arrow reflects the magnitude of the force. • The direction of the arrow shows the direction that the force is acting. • Each force arrow in the diagram is labeled to indicate the exact type of force. • In a free-body diagram the object is represented by a box or dot

  11. Free Body Diagram Practice • A book is at rest on a tabletop. Diagram the forces acting on the book. • A girl is suspended motionless from the ceiling by two ropes. Diagram the forces acting on the girl. • An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it is falling. • A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel. • A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.

  12. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. • A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. Diagram the vertical forces acting on the backpack. • A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver. • A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration. Neglect air resistance. Diagram the forces acting upon the sled. • A football is moving upwards towards its peak after having been booted by the punter. Neglect air resistance. Diagram the forces acting upon the football as it rises upward towards its peak. • A car is coasting to the right and slowing down. Neglect air resistance.

  13. C. Friction

  14. So far, How have we determined the net force on an object? • It is the sum of all forces • Fnet = ΣF

  15. What if forces are working directly opposite of one another? • What can we say about the object’s velocity? • Velocity is constant; forces balanced. Can be 0m/s or greater. • What can we say about the object’s acceleration? • Acceleration is 0. 100 N 100 N 25 kg

  16. What if forces are working directly opposite of one another? • What can we say about the object’s velocity? • Velocity is changing; forces unbalanced. • What can we say about the object’s acceleration? • Acceleration is to the right. Net force is 100N right • a = F/m • a = 100N/25 kg 4m/s2 200 N 100 N 25 kg

  17. A box is sitting on a table. What forces are working on it? • Why doesn’t it float above the table? • Why doesn’t it sink through the table? • What happens if I try to slide the box left or right? Now what forces are working on it?

  18. When Surfaces Pressed Together there are 4 Forces • Force of Weight or Gravity: Fw • Normal Force: FN • Friction Force: FF • Applied Force: FA

  19. Surfaces Pressed Together • Normal Force: FN • Usually a 3rd Law reaction to gravity, that is equal and opposite of Force of Weight (Fw)**** • Perpendicular to the surface.

  20. Would you like to ride this bike?

  21. Friction • Friction is a force that is created whenever two surfaces move or try to move across each other.  • Friction always opposes the motion or attempted motion of one surface across another surface.

  22. Friction • Depends on • the texture of both surfaces. • amount of contact force pushing the two surfaces together • Static: • Object is not moving • Kinetic: • Object is moving

  23. How does friction affect net force? Fne t =FA+ -FF Fnet = ma • FA + - FF = ma Where: • FF = Friction Force m = mass • FA = Applied Force a = acceleration • Ff is negative because it is in the opposite direction!

  24. FNET = FN

  25. Ex A • A 10-kg object travels along a horizontal surface at a constant velocity of 10 m/s. The force of friction is 20 N. Draw a free body diagram showing Fw, FN, and Ff. • What is the FNET • What is the FA

  26. Ex A • A 10-kg object travels along a horizontal surface at a constant velocity of 10 m/s. The force of friction is 20 N. Draw a free body diagram showing Fw, FN, FA and Ff. • What is the FNET ? • 0 no acceleration • What is the FA? • FA = FF • Balanced Forces FN 98 N FA 20 N FF 20 N 10 kg FW 98 N

  27. Ex B • A 10-kg object accelerates along a horizontal surface at 5 m/s2. The force of friction is 20 N. Draw a free body diagram showing Fw, FN, and Ff. • What is the FNET • Calculate FA

  28. Ex B • A 10-kg object accelerates along a horizontal surface at 5 m/s2. The force of friction is 20 N. Draw a free body diagram showing Fw, FN, FA and Ff. • What is the FNET • FNET =(10kg)(5 m/s2) = 50 N • Calculate FA • FA = FNET + FF FN 98 N FF 20 N FA 70 N 10 kg FW 98 N

  29. What is another term for an unbalanced force? • Net Force • What happens if an object experiences an unbalanced force? • Its velocity changes (accelerates)

  30. Is friction the same between all surfaces? • Coefficient of Friction Reference Table - Engineer's Handbook • The coefficient of friction (static or kinetic) is a measure of how difficult it is to slide a material of one kind over another; the coefficient of friction applies to a pair of materials, and not simply to one object by itself.

  31. Surfaces Pressed Together • Coefficent of Friction µ can be calculated • It is a ratio of FF and FN

  32. I call these FAWN problems

  33. Sample Problems • First without ftiction……….

  34. Example C: Let’s consider a stationary crate on a horizontal, frictionless plane. The mass of this crate is 114.38 kg. What is its weight (FW)? What is the normal force (FN)? What applied force (FA) is necessary to accelerate the crate horizontally at 1.08 m/s2?

  35. FN 114.38kg Fw

  36. 1120N 114.38kg 1120N

  37. What FA to accelerate 1.08m/s2? • FA = ma • FA = (114.38 kg)(1.08m/s2) • FA = 124 N FA? 114.38kg

  38. Example C: A stationary crate on a horizontal plane. The mass of this crate is 114.38 kg. The μkinetic between the crate and surface is 0.0258. The μstatic between the crate and surface is 0.0354 What is the horizontal FA required to just get the crate to move? Did you notice in your lag how the newton scale jumped right at your box started to move? You were seeing the difference between static and kinetic force.

  39. First get it to move…. FN 1120N FA FF 114.38kg Which μ to use? STATIC!!! FW 1120N

  40. FA = FF if velocity zero • FF = (0.0354) (1120 N) • FF = 39.6 N • Since FA = FF then the FA = 39.6 N to get the crate to move • Example D. The object is now moving at a constant velocity. Do you think the FA to keep the crate moving will be more or less than the FA to get it moving?

  41. EX. D • FF = (0.0258) (1120 N) • FF = 28.9 N • Since FA = FF then the FA = 28.9 N

  42. Extra practice E: A crate is sliding across a horizontal plane. The mass of this crate is 114.38 kg. The μkinetic between the crate and surface is 0.0258. The μstatic between the crate and surface is 0.0354. What is the horizontal FA required to get the crate to accelerate at 2.5 m/s2? .

  43. Extra Practice F A 50 kg object accelerates horizontally at 0.2 m/s2 from rest for 5 seconds. If the coefficient of friction is 0.01, what is the Fnet? What is the FF ? What is the FA ?How far does it travel in 5 seconds?:

  44. Ex. G This same crate is now going to be pulled with a rope across a frictionless horizontal surface. The rope makes an angle of θ to the floor. 114.38kg θ

  45. TENSION aka FT • is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. • It is the opposite of compression. It is a “response force” • That is to say, if one pulls on the rope, the rope fights back by resisting being stretched • Ropes, strings, and cables can only pull. They cannot push because they bend. • is measured in newtons • is always measured parallel to the string on which it applies.

  46. What does the rope provide? A lift (vertical force) and a pull (horizontal force) If there was no angle, would there be any vertical force? No If the angle was at 90°, how would that affect the force components? Force would only be in the vertical plane How would you calculate the horizontal and vertical force components if the angle of the rope with the floor was 57.3° and the Force of tension (FA) in the rope was 801 N?

  47. G. A box is pulled into motion with a rope across a horizontal surface. The rope makes an angle of 57.3° to the floor. The Force of tension (FA)in the rope is 801N FAY or FVert 801 N 114.38kg 57.3° FAX or FHoriz B. Determine FAY or FVert = sin (57.3°) (801N) = 674 N A. Determine FAX or FHoriz = cos (57.3°) (801N) = 433 N

  48. C. If this object is pulled into horizontal motion, what is its initial velocity? 0 m/s D. What is its horizontal acceleration? FAX = ma REMEMBER: Acceleration is in the direction of the force. 433N/ 114.38kg = 3.79 m/s2 E. What is its final velocity at a distance of 38.7 meters? vf2 = vi2 + 2ad vf2 = (0 m/s)2 + [(2)(3.79 m/s2)(38.7 m)] vf =17.1 m/s F. Did the determination of FA,Y serve any purpose in this particular Example? Explain. No. It is not needed in the example.

  49. Ex H. This same crate is now going to be pulled with a rope across a friction based horizontal surface. The coefficient of friction between the crate and the surface is 0.2534. The angle remains 57.3° and the rope exerts a tension of 801 N 114.38kg 57.3°

  50. H. A box is pulled into motion with a rope across a horizontal surface. The rope makes an angle of 57.3° to the floor. The Force of tension (FA)in the rope is 801N FAY or FVert 801 N 114.38kg 57.3° FAX or FHoriz B. Determine FAY or FVert = sin (57.3°) (801N) = 674 N A. Determine FAX or FHoriz = cos (57.3°) (801N) = 433 N

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