Day 7

Friday, January 25, 2008

Announcements:

Assignments:

Answer the recommended conceptual exercises and problems for Chapter 4 (listed at the bottom of the daily schedule), and check your understanding by looking at the answers in the back of the textbook.

Class Discussion:

Chapter 3 continued

Demo: Ramp with blinking lights
    What is the acceleration of the balls rolling down the ramp?
    If a ball is pushed up the ramp from the bottom, what will be its acceleration as it moves up to the top and then down the ramp?
    Sketch graphs of position, velocity, and acceleration as functions of time for this scenario.

Draw a graph of vertical velocity as a function of time for a ball thrown up to a height of 5 m and then caught. How does this graph compare with the velocity graph for the ball and ramp?

Free-fall occurs when an object falls without any external forces other than gravity (i.e. no air resistance). Many falling objects experience nearly free-fall conditions when air resistance is negligible. Examples?
Video of hammer and feather drop on moon: http://www.teachersdomain.org/resources/phy03/sci/phys/mfw/galmoon/index.html
Demo: penny and feather in vacuum, book and paper drop (example of "drafting")

Useful Kinematics Equations:
    The final speed of an object moving with constant acceleration is: vf = 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

How can a meter stick be used to measure a person's reaction time?
Min-lab: Determine your reaction time using only a meter stick as a measuring device.
    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: falling sticks

Demo: falling water drops

Chapter 4 - Why things move

How is Newtonian thinking different from thought that preceeded Isaac Newton?
Newton's Laws of motion:
    1) Inertia: A net external force is required to change an object's velocity.
    2) F = ma      Why does this equation make more sense written: a = F/m?
    3) Action/Reaction: F₁₂ = -F₂₁   Equal and opposite forces on different objects.
Definition of mass - measure of inertia, resistance to acceleration; measured in kilograms (kg)
Definition of force - a push or pull, measured in Newtons (N). 1 lb. ~ 4 N
Draw a graph for the acceleration that results from force applied to a mass (a vs. F). What does the slope represent?

Minute Paper: Write you name on a piece of paper, and hand to another student who will answer the questions below.
1) What is the most interesting thing you learned in class today?
2) What questions or concerns do you have?
3) What feedback (positive and negative) can you offer about today's lecture?