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PHYSICS 50: Lecture 5.1

This lecture discusses the concepts of friction, forces, and their impact on motion. Topics include examples of friction, understanding the free-body diagram, and problem-solving exercises. Readings and homework assignments are given.

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PHYSICS 50: Lecture 5.1

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  1. PHYSICS 50: Lecture 5.1 RICHARD CRAIG

  2. Goals for Today • Quiz Review • Homework • Example of understanding questions • New topic: Friction • Examples • Car on a curved track

  3. Homework #4 • Read Chapter 5: Sections 1-4 • Exercises and Problems: 5.5, 5.18, 5.34, 5.37, 5.52 • Due Thursday, 2/21

  4. What is a Force? • A push or a pull • It has both a magnitude and a direction (vector) • It has units of N (Newton) = kgm/s2

  5. Q5.1 A car engine is suspended from a chain linked at O to two other chains. Which of the following forces should be included in the free-body diagram for the engine? A. tension T1 B. tension T2 C. tension T3 D. two of the above E. all of T1, T2, and T3

  6. Q5.2 A cable attached to a car holds the car at rest on the frictionless ramp (angle a). The ramp exerts a normal force on the car. How does the magnitude n of the normal force compare to the weight w of the car? A. n = w B. n > w C. n < w D. not enough information given to decide

  7. Q5.3 A cart (weight w1) is attached by a lightweight cable to a bucket (weight w2) as shown. The ramp is frictionless. When released, the cart accelerates up the ramp. Which of the following is a correct free-body diagram for the cart?0 n n n n T T T T m1a m1a w1 w1 w1 w1 A. B. C. D.

  8. Q5.5 A lightweight crate (A) and a heavy crate (B) are side-by-side on a frictionless horizontal surface. You are applying a horizontal force F to crate A. Which of the following forces should be included in a free-body diagram for crate B? A. the weight of crate B B. the force of crate B on crate A C. the force F that you exert D. the acceleration of crate B E. more than one of the above F B A

  9. Frictional forces, kinetic and Static • Friction can keep an object from moving or slow its motion from what we last calculated on an ideal, frictionless surface. • Microscopic imperfections cause non-ideal motion.

  10. Coefficients of friction

  11. Applied force and Frictional Force • Notice the transition between static and kinetic friction.

  12. Friction Examples • Book on Table (easy) • Pulling Sled (medium) • Blocks on an Incline (hard)

  13. Q5.11 A car (mass m) moves at a constant speed v around a flat, unbanked curve of radius R. A free-body diagram for the car should include A. an outward centrifugal force of magnitude mv2/R. B. an inward centripetal force of magnitude mv2/R. C. the force of the car’s acceleration. D. two of the above. E. none of the above.

  14. Q5.12 A car (mass m) moves at a constant speed v around a banked curve of radius R and bank angle b (measured from the horizontal). A free-body diagram for the car should include A. a normal force that points vertically upward. B. a normal force that points at an angle b from the vertical. C. a normal force that points at an angle b from the horizontal. D. an outward centrifugal force of magnitude mv2/R. E. more than one of the above.

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