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

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

3. 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 Administration

4. Review Questions Lets 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+v 0, what does x=? – x=(1/2)at 2 +v 0 t+x 0

7. Review Questions When you throw a ball up, at max height, acceleration in y is? (-9.8 m/s 2 ) 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 cars acceleration?(8 m/s 2 )

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

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. 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 XY

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 objects motion Motion of object out of frame is equal to objects 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 cant 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? 5 m/s Original rain 5 m/s Wind at 2 m/s Scenario #1 Scenario #2

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