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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

PHYS16 – Lecture 6

Physics of 2D Motion

September 20, 2010

To run or walk???

agenda
Agenda
  • Administration
  • Key Concepts – Physics 2D motion
  • Relative Motion
administration
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
review questions
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

?

review questions1
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
review questions2
Review Questions
  • v=at+v0 , what does x=?
    • x=(1/2)at2+v0t+x0
review questions3
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)
usain bolt vs a fly
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

usain bolt vs fly
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

key concepts physics 2d motion
Key Concepts: Physics 2D Motion
  • 2D Motion
    • Independent in x and y
    • Constant acceleration
  • Relative Motion
  • Projectile Motion
2d motion1
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?
relative motion math
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
relative motion walkway at airport
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
relative motion river problem
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

relative motion river problem1
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
relative motion bucket problem
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

relative motion run or walk in rain
Relative Motion – Run or Walk in Rain?
  • Is it better to run or walk in the rain?
relative motion run or walk in rain1
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
summary
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