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Forces: Friction and Tension

12 September 2013. Forces: Friction and Tension. Agenda Projectile motion practice follow-up Pulley HW problem Forces in two dimensions Coefficients of friction Inclined planes Essential Question: How do friction and inclines affect the motion of objects?.

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Forces: Friction and Tension

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  1. 12 September 2013 Forces: Friction and Tension

  2. Agenda • Projectile motion practice follow-up • Pulley HW problem • Forces in two dimensions • Coefficients of friction • Inclined planes • Essential Question: How do friction and inclines affect the motion of objects?

  3. Exam 1: Friday, September 20th • Vectors • Motion graphs • Free Fall • Projectile Motion • Newton’s Laws and 1-D forces • Review Day: Thursday (Half-day) • Additional review time (?)

  4. Projectile Motion Practice • We all got through Problem 1 together • Let’s go over Problem 2

  5. Pulley Problem Draw free body diagram for both masses What is the acceleration of the system? What is the tension in the rope?

  6. Forces: 2D • Consider a block of mass 2 kg hanging from three cords where a = 45° and B = 60°. What are the tensions in the three cords? • Step 1: Draw a free body diagram for the place where the three cords meet

  7. Remember to add a coordinate system

  8. Forces: 2D • You pull a 21 kg suitcase at a constant velocity by applying a force of 120 N at an angle of 40° above the horizontal. Determine the normal force and the friction force acting on the suitcase.

  9. Forces: 2D • A Northampton banner weighs 45 kg and hangs from two cables that make an angle of 35° with the horizontal. What is the tension in the two cables?

  10. More on Friction • Static friction: Prevents on object from sliding (the object is not moving) • s is the coefficient of static friction • N is the normal force • Kinetic friction: Slows down a moving object • k is the coefficient of kinetic friction • N is the normal force

  11. y FBD for box: Nsb Fk;sb x web Example: A box slides across a rough surface. If the coefficient of kinetic friction is 0.3, what is the acceleration of the box? Apply Newton’s 2nd Law:

  12. Inclines • Problems with a surface that is not level • Coordinate Axes • Standard axes will make problem more difficult • Rotate axes so that x-axis aligns with plane • Will require finding components of gravity that match up with new axes

  13. FBD for box y Nrb fs;rb  web x  To make things more complicated… Example: A 1.00 kg mass is at rest an on ramp that makes an angle of 20 with respect to the horizontal. For this situation s = 0.400. What is the magnitude of the static friction force? Apply Newton’s 2nd law

  14. Inclines (1) (2) The magnitude of the static friction force can be found from (1) What is the magnitude of the maximum static friction force?

  15. Inclines A car weighing 1.2 x 104N is parked on a hill with a slope of 36 degrees How great is the force down the hill? What is the normal force? If the car’s parking brake fails, at what rate will it accelerate (assume negligible friction)

  16. Tomorrow… • Inclined plane lab!

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