sy02 sept10 07hc

Slide1: 

Physics 207 Labs……start this week (MC1a & 1c)

Physics 207, Lecture 2, Sept. 10: 

Physics 207, Lecture 2, Sept. 10 Agenda for Today Finish Chapter 1, Chapter 2.1, 2.2 Units and scales, order of magnitude calculations, significant digits (on your own for the most part) Position, Displacement Velocity (Average and Instantaneous), Speed Acceleration Dimensional Analysis Assignments: For next class: Finish reading Ch. 2, read Chapter 3 (Vectors) Mastering Physics: HW1 Set due this Wednesday, 9/10 Mastering Physics: HW2 available soon, due Wednesday, 9/17 (Each assignment will contain, 10 to 11 problems

Length: 

Length Distance Length (m) Radius of Visible Universe 1 x 1026 To Andromeda Galaxy 2 x 1022 To nearest star 4 x 1016 Earth to Sun 1.5 x 1011 Radius of Earth 6.4 x 106 Sears Tower 4.5 x 102 Football Field 1 x 102 Tall person 2 x 100 Thickness of paper 1 x 10-4 Wavelength of blue light 4 x 10-7 Diameter of hydrogen atom 1 x 10-10 Diameter of proton 1 x 10-15

Time: 

Time Interval Time (s) Age of Universe 5 x 1017 Age of Grand Canyon 3 x 1014 Avg age of college student 6.3 x 108 One year 3.2 x 107 One hour 3.6 x 103 Light travel from Earth to Moon 1.3 x 100 One cycle of guitar A string 2 x 10-3 One cycle of FM radio wave 6 x 10-8 One cycle of visible light 1 x 10-15 Time for light to cross a proton 1 x 10-24

Mass: 

Mass Object Mass (kg) Visible universe ~ 1052 Milky Way galaxy 7 x 1041 Sun 2 x 1030 Earth 6 x 1024 Boeing 747 4 x 105 Car 1 x 103 Student 7 x 101 Dust particle 1 x 10-9 Bacterium 1 x 10-15 Proton 2 x 10-27 Electron 9 x 10-31 Neutrino <1 x 10-36

Some Prefixes for Power of Ten: 

Some Prefixes for Power of Ten Power Prefix Abbreviation 103 kilo k 106 mega M 109 giga G 1012 tera T 1015 peta P 1018 exa E 10-18 atto a 10-15 femto f 10-12 pico p 10-9 nano n 10-6 micro m 10-3 milli m

Order of Magnitude Calculations / Estimates Question: How many french fries, placed end to end, would it take to reach the moon? : 

Order of Magnitude Calculations / Estimates Question: How many french fries, placed end to end, would it take to reach the moon? Need to know something from your experience: Average length of french fry: 3 inches or 8 cm, 0.08 m Earth to moon distance: 250,000 miles In meters: 1.6 x 2.5 X 105 km = 4 X 108 m

Dimensional Analysis : 

This is a very important tool to check your work Provides a reality check (if dimensional analysis fails then no sense in putting in the numbers; this leads to the GIGO paradigm) Example When working a problem you get the answer for distance d = v t 2 ( velocity x time2 ) Quantity on left side = L Quantity on right side = L / T x T2 = L x T Left units and right units don’t match, so answer is nonsense Dimensional Analysis

Lecture 2, Exercise 1Dimensional Analysis : 

Lecture 2, Exercise 1 Dimensional Analysis The force (F) to keep an object moving in a circle can be described in terms of: velocity (v, dimension L / T) of the object mass (m, dimension M) radius of the circle (R, dimension L) Which of the following formulas for F could be correct ?

 Lecture 2, Exercise 1Dimensional Analysis Which of the following formulas for F could be correct ?: 

Lecture 2, Exercise 1 Dimensional Analysis Which of the following formulas for F could be correct ?    Note: Force has dimensions of ML/T2 Velocity (n, dimension L / T) Mass (m, dimension M) Radius of the circle (R, dimension L)

Converting between different systems of units: 

Converting between different systems of units Useful Conversion factors: 1 inch = 2.54 cm 1 m = 3.28 ft 1 mile = 5280 ft 1 mile = 1.61 km Example: Convert miles per hour to meters per second:

Lecture 2, Home Exercise 1 Converting between different systems of units: 

Lecture 2, Home Exercise 1 Converting between different systems of units When on travel in Europe you rent a small car which consumes 6 liters of gasoline per 100 km. What is the MPG of the car ? (There are 3.8 liters per gallon.)

Significant Figures: 

Significant Figures The number of digits that have merit in a measurement or calculation. When writing a number, all non-zero digits are significant. Zeros may or may not be significant. those used to position the decimal point are not significant (unless followed by a decimal point) those used to position powers of ten ordinals may or may not be significant. In scientific notation all digits are significant Examples: 2 1 sig fig 40 ambiguous, could be 1 or 2 sig figs (use scientific notations) 4.0 x 101 2 significant figures 0.0031 2 significant figures 3.03 3 significant figures

Significant Figures: 

Significant Figures When multiplying or dividing, the answer should have the same number of significant figures as the least accurate of the quantities in the calculation. When adding or subtracting, the number of digits to the right of the decimal point should equal that of the term in the sum or difference that has the smallest number of digits to the right of the decimal point. Examples: 2 x 3.1 = 6 4.0 x 101 / 2.04 x 102 = 1.6 X 10-1 2.4 – 0.0023 = 2.4

Motion in One-Dimension (Kinematics) Position / Displacement: 

Motion in One-Dimension (Kinematics) Position / Displacement Position is usually measured and referenced to an origin: At time= 0 seconds Joe is 10 meters to the right of the lamp origin = lamp positive direction = to the right of the lamp position vector : 10 meters

Position / Displacement: 

Position / Displacement One second later Joe is 15 meters to the right of the lamp Displacement is just change in position. x = xf - xi 10 meters O xf = xi + x x = xf - xi = 5 meters t = tf - ti = 1 second xf

Average speed and velocityChanges in position vs Changes in time: 

Average speed and velocity Changes in position vs Changes in time Speed is just the magnitude of velocity. The “how fast” without the direction. Average velocity = total distance covered per total time,

Average Velocity Exercise 2 What is the average velocity over the first 4 seconds ?: 

Average Velocity Exercise 2 What is the average velocity over the first 4 seconds ? 2 m/s 4 m/s 1 m/s 0 m/s x (meters) t (seconds) 2 6 -2 4 1 2 4 3

Average Velocity Exercise 3 What is the average velocity in the last second (t = 3 to 4) ?: 

Average Velocity Exercise 3 What is the average velocity in the last second (t = 3 to 4) ? 2 m/s 4 m/s 1 m/s 0 m/s x (meters) t (seconds) 2 6 -2 4 1 2 4 3

Instantaneous velocity Exercise 4 What is the instantaneous velocity in the last second?: 

Instantaneous velocity Exercise 4 What is the instantaneous velocity in the last second? -2 m/s 4 m/s 1 m/s 0 m/s x (meters) t (seconds) 2 6 -2 4 1 2 4 3

Average Speed Exercise 5 What is the average speed over the first 4 seconds ?: 

Average Speed Exercise 5 What is the average speed over the first 4 seconds ? 2 m/s 4 m/s 1 m/s 0 m/s x (meters) t (seconds) 2 6 -2 4 1 2 4 3 turning point

Key point:: 

Key point: If the position x is known as a function of time, then we can find both velocity v Area under the v(t) curve yields the change in position Algebraically, a special case, if the velocity is a constant then x(t)=v t + x0

Exercise 6, (and some things are easier than they appear): 

Exercise 6, (and some things are easier than they appear) A marathon runner runs at a steady 15 km/hr. When the runner is 7.5 km from the finish, a bird begins flying from the runner to the finish at 30 km/hr. When the bird reaches the finish line, it turns around and flies back to the runner, and then turns around again, repeating the back-and-forth trips until the runner reaches the finish line. How many kilometers does the bird travel? A. 10 km B. 15 km C. 20 km D. 30 km

Motion in Two-Dimensions (Kinematics) Position / Displacement: 

Motion in Two-Dimensions (Kinematics) Position / Displacement Amy has a different plan (top view): At time= 0 seconds Amy is 10 meters to the right of the lamp (East) origin = lamp positive x-direction = east of the lamp position y-direction = north of the lamp 10 meters

Motion in Two-Dimensions (Kinematics) Position / Displacement: 

Motion in Two-Dimensions (Kinematics) Position / Displacement At time= 1 second Amy is 10 meters to the right of the lamp and 5 meters to the south of the lamp O -x +x -y +y

Position, velocity & acceleration: 

Position, velocity & acceleration All are vectors! Cannot be used interchangeably (different units!) (e.g., position vectors cannot be added directly to velocity vectors) But the directions can be determined “Change in the position” vector gives the direction of the velocity vector “Change in the velocity” vector gives the direction of the acceleration vector Given x(t)  v(t)  a(t) Given a(t)  v(t)  x(t)

And given a constant acceleration we can integrate to get explicit v and a: 

And given a constant acceleration we can integrate to get explicit v and a x a v t t t

Assignment Recap: 

Assignment Recap Reading for Wednesday’s class on 9/12 Finish Chapter 2 (gravity & the inclined plane) Chapter 3 (vectors) And first assignment is due this Wednesday