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PHYS16 – Lecture 6

PHYS16 – Lecture 6. Physics of 2D Motion September 20, 2010. To run or walk???. Agenda. Administration Key Concepts – Physics 2D motion Relative Motion. Administration. Homework Assignments Due Tuesday, Online lab homework Due Sunday, Written homework, & Ch. 4 Online homework

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PHYS16 – Lecture 6

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  1. PHYS16 – Lecture 6 Physics of 2D Motion September 20, 2010 To run or walk???

  2. Agenda • Administration • Key Concepts – Physics 2D motion • Relative Motion

  3. Administration • Homework Assignments • Due Tuesday, Online lab homework • Due Sunday, Written homework, & Ch. 4 Online homework • Help Sessions – T/TH 8-9pm • View Jupiter – Tonight 9 pm, Merrill Beach

  4. Review Questions • Let’s say you have a velocity vector that is 6 mph at 55 degrees relative to the x axis. What is the component of the vector along x? • A) 6cos(55 degrees) • B) 6sin(55 degrees) • C) 6/sin(55 degrees) • D) 6tan(55 degrees) 6 mph ?

  5. Review Questions • If you add vector 1 (5 at 37 degrees) to vector 2 (2 at 0 degrees), what do you get for the component in the x direction? • A) 5 • B) 4 • C) 6 • D) 7

  6. Review Questions • v=at+v0 , what does x=? • x=(1/2)at2+v0t+x0

  7. Review Questions • When you throw a ball up, at max height, acceleration in y is? (-9.8 m/s2) • When you throw a ball up, at max height, velocity in y is? (0 m/s) • A car goes from 0 to 4 m/s in 0.5 s. What is the car’s acceleration? (8 m/s2)

  8. Usain Bolt vs. a Fly Usain’s Position vs. time Except between time 0 s to 1.89 s, Usain has a constant velocity of ~10 m/s Usain’s acceleration = 0 m/s^2

  9. Usain Bolt vs. Fly Position vs. time for two flies Each dot is a position at 20 Hz Velocity vs. time and acceleration vs. time is more complicated than Usain Bolt

  10. Key Concepts: Physics 2D Motion • 2D Motion • Independent in x and y • Constant acceleration • Relative Motion • Projectile Motion

  11. 2D Motion

  12. 2D Motion • Motion in x and motion in y is independent • Separate into motion in x and motion in y • Use the equations of motion for 2D to solve for unknowns • Separate and Solve • Most problems will assume constant acceleration • Equations of motion?

  13. Kinematic Equations X Y

  14. Relative Motion

  15. Relative Motion – Math • Relative motion – motion of a frame that an object is moving in • Motion of object in frame is just equal to object’s motion • Motion of object out of frame is equal to object’s motion + frame motion

  16. Relative Motion – Walkway at Airport • I am running at 2 m/s in the airport and the moving walkway is at 3 m/s. If I take the walkway what is my speed? • 5 m/s • Do I take the walkway or run if there is a family on walkway so that I can’t run on it? • take walkway • If I just run the 10 m to the gate instead of taking the walkway will I still beat my brother who is going 3 m/s and is 11 m behind me? • yes

  17. Relative Motion – River Problem • A river has a current of 3 m/s. A boat pointed straight across the river with a speed of 4 m/s appears from the shore to have a velocity of what value? 4 m/s 3 m/s

  18. Relative Motion – River Problem • A river has a current of 3 m/s. A boat pointed straight across the river with a speed of 4 m/s appears from the shore to have a velocity of what value? • 5 m/s at an angle of 40 degrees from the shore

  19. Relative Motion – Bucket Problem • A bucket is left out in the rain. The rain is coming down at 5 m/s. If a crosswind starts to blow at 2 m/s, will the bucket fill faster or slower? Wind at 2 m/s Original rain 5 m/s 5 m/s Scenario #2 Scenario #1

  20. Relative Motion – Run or Walk in Rain? • Is it better to run or walk in the rain?

  21. Relative Motion – Run or Walk in Rain? • Is it better to run or walk in the rain? • Assume rain is coming straight down at 1 m/s • If you run at 2 m/s, what does the rain look like in your frame? • looks like coming at you instead of straight down • If you just look at surface area of body getting wet, does walking or running hit less surface area? • Walking • If you look at how many rain drops hit you, does walking or running have less total rain drops? • Running because you reach your destination sooner

  22. Summary • Separate 2D motion into x and y and then solve • Use Kinematic equations of Motion to solve for problems with constant acceleration • Relative Motion – make sure to add velocity of frame to velocity of object to get relative velocity

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