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Common Forces

Common Forces. Physics 12. Joke of the day:. Not physics but still funny?. Review: Free-body diagrams:. Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object. What if some of the forces were not in the x or y plane???????.

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Common Forces

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  1. Common Forces Physics 12

  2. Joke of the day: • Not physics but still funny?

  3. Review: Free-body diagrams: • Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object. • What if some of the forces were not in the x or y plane???????

  4. Forces at angles: • We will break the forces with angles into their x and y components using trigonometry! • AGAIN! Fy = F sin θ Fx = F cos θ

  5. Example #1: • A 35kg sign is suspended by two wires that each make a 25° with the horizontal. Calculate the tension in each wire. • Draw a FBD. 25° 25° 35kg

  6. Free Body Diagram • What do we know? What can we calculate? Φ=155° Θ=25° Fg

  7. Net Force: • Not moving….so no acceleration • Thus….

  8. X components: Is there Fg? No! Others: Y components: Is there Fg ? Yes! (Fgy ) Others: Components: So what does that tell us?

  9. Analysis: • X-direction

  10. Analysis • Y-direction

  11. Analysis (almost done!):

  12. Review: Normal force • is perpendicular to the surface of contact • contact force exerted on an object • For example, the surface of a floor or wall, preventing the object from penetrating the surface.

  13. Review: Force of friction: • Direction of Ff is always opposite to the direction of motion Ff = µ FN • µ is the coefficient of friction (“stickiness value”, no units)

  14. Example #2: Ff • Holding a Book Against a Wall • What is the applied force? FN FA Fg

  15. Holding a Book Against a Wall Ff FN FA Fg

  16. Try it : • Page 467 • Questions 8,9

  17. Day 2: • Inclined planes

  18. Inclined Planes • Label the direction of FN and Fg. N θ mg Not both in the x or y plane…….

  19. Inclined Planes • Take note of and mark the direction of acceleration (a). N a θ mg

  20. Inclined Planes y • Choose the coordinate system with x in the same or opposite direction of acceleration and y perpendicular to x. • Draw it on the diagram!!!!!!! N x a θ mg

  21. Inclined Planes y • Now some trigonometry! N x a θ 90- θ mg θ

  22. Inclined Planes y • Find the components of the force of gravity. N x a θ θ mg

  23. Example #3: • A worker places a large plastic waste container with a mass of 84 kg on the ramp of a loading dock as shown in the figure. The ramp makes an angle of 22° with the horizontal. The workers turns to pick up another container before pushing the first one up the ramp. (a) If the coefficient of static friction is 0.47, will the crate slide down the ramp? (b) If the crate does slide down, what will be its acceleration?

  24. Try it  • Page 474 • 10, 11, 12, 13(b)

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