Thursday, May 12, 2011

Announcements:

• Labs begin today in Phillips 206.
• Fill out a Student Survey if you have not yet done so.
• If your name appears on the screen, register your clicker by following the three steps.

Assignments:

• Submit HW2 by midnight tonight
• Read Chapter 3 in your textbook and submit HW3 by midnight tomorrow.
• Review answers to Measurement Uncertainty Quiz.  Ask your TA or professor for clarification if necessary.

CHAPTER 2: Motion in One Dimension

Position is the location of an object relative to some point in space.
Displacement (a vector) is the change in position of an object: final position - initial position
Distance is the total length traversed by an object.
Average speed = distance/elapsed time
Average velocity = displacement/elapsed time
(Instantaneous) velocity:  v = dx/dt
Speed (a scalar) is the magnitude of the velocity (a vector)
Average acceleration = change in velocity/change in time
(Instantaneous) acceleration:  a = dv/dt
The magnitude of the acceleration does not have a special name.
The change in acceleration is called a "jerk" = da/dt

Equations of motion for constant acceleration: (see Table 2-4 on p.34)
Note:  These equations come from integrating and the definitions of velocity and acceleration

Example:  Walk forward 3 m and back 3 m in 6 seconds.  What are the above quantities?

Exercise:  A car accelerates from 0 to 30 m/s in 6 seconds
 - Sketch the position versus time graph
 - Sketch the velocity versus time graph
 - Sketch the acceleration versus time graph

Useful Kinematics Equations:
    The final speed of an object moving with constant acceleration is:  vf = vo + a*t
    Distance traveled for object moving with constant acceleration:  d = 1/2a*t^2 + vo*t
    Combining the above equations and eliminating the time gives:  (vf)^2 = (vo)^2 + 2*a*d

Problem 2.71 - Measuring reaction time with a ruler
    Questions to consider:  What are some common situations where reaction time is important?  Classify these scenarios according to type:  unexpected, expected, poised, predicted, choice (Go/NoGo), other.
    Typical human reaction times are 200 to 230 ms  (Ref. Wikipedia)
    You can test your reaction time online:  http://getyourwebsitehere.com/jswb/rttest01.html

Demo: Catch a falling dollar
Demo: Falling water drops with strobe light
Demo:  Falling in vacuum (paper on top of book)
Demo: Feather and hammer video on moon

Demo: What is the acceleration of the ball rolling down the ramp with lights that flash at 1 Hz?   (Data: x1 = 1 ft., x2 = 4 ft., x3 = 9 ft.)
If a ball is pushed up this ramp, what will be its acceleration as it moves up and then down the ramp?

Draw graphs of vertical position, velocity, and acceleration as functions of time for a ball thrown up to a height of 5 m and then caught at the initial level.

Chapter 2 ConceptTests using clickers

Chapter 3:  Vectors

Vectors and components: 

Vectors A and B have the same magnitude and orientation.  Does A = B?

Vector A is 10 m long and oriented 30 degrees above the positive x axis.
(T/F):  A = A_x + A_y

A = A_xx + A_yy
A = Acos30x + Asin30y
A = Asin60x + Acos60y

The top of a tree is 30 degrees above horizontal when viewed from a distance of 50 feet from its base.  What is the approximate height of this tree?
A)  20 ft.    B)  30 ft.    C)  40 ft.    D)  50 ft.    E)  60 ft.

Methods for vector addition:
 - Graphically using tail-to-head or parallelogram method
 - Components or unit-vector notation

Example problem:  An airplane flies with an airspeed of 100 m/s at a compass heading of 120 degrees.  What is the displacement of this plane after 5 minutes, assuming there is no wind?  What if there is wind from the north at 20 m/s?

Chapter 3 ConceptTests using clickers