HTWClass4F07

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Unformatted text preview: Register/re-register i-clicker Need to re-register: Lane, Charles Kenneth pick one from the list use it during the class register afterwards A6 B5 D4 E5 E6 F2 F7 K2 K5 M1 N4 Quezada, Andres Luca Rain, Caitlin Evans Simmons, Avery K West, Brett Thomas Done after class Physics 100 Fall 2007 Schedule Monday 8/27 Pretest 1 due Pretest 2 posted Monday 9/3 Labor Day No Class Pretest 3 posted Tuesday Tuesday Wednesday 8/29 HW 1 due HW 2 posted Wednesday 9/5 HW 3 posted HE 1 posted Thursday Thursday Friday 8/31 Pretest 2 due Friday 9/7 HW 2 due Physics 100 Fall 2007 Online Resources How Things Work Cases For cases, see the HTW Student Companion Website This is also available via External Links on the Course Blackboard site. Physics 100 Fall 2007 A constant force is exerted on a "skater" that is initially at rest on "ice" an air track. The force acts for a short time interval and gives her a certain final speed. "skater" To reach the same final speed with a force that is only half as big, the force must be exerted for a time interval A. four times as long as B. twice as long as C. equal to D. half as long as E. a quarter of that for the stronger force. Physics 100 Fall 2007 Answer: B Calculations with Unit Conversion Body Mass Index (BMI) = (mass [kg])/(height [m])2 To calculate BMI using English units of pounds-mass and inches, one converts each unit by multiplying by factors of magnitude one, as shown below: (kg ) mass(lbm) 1kg 12 2 in 2 3.28082 ft 2 BMI 2 = (m ) height 2 (in 2 ) 2.205lbm 12 ft 2 12 m 2 kg mass in pounds = 702.91 2 m (height in inches) 2 ? Physics 100 Fall 2007 Note that you can check your method by canceling Falling Balls Section 1.2 Physics 100 Fall 2007 Introductory Question Suppose I throw a ball upward into the air. After the ball leaves my hand, is there any force pushing the ball upward? A. Yes B. No Physics 100 Fall 2007 Answer B Observations about Falling Balls When you drop a ball, it begins at rest, but acquires downward speed covers more and more distance each second When you tossed a ball straight up, it rises to a certain height comes briefly to a stop begins to descend, much like a dropped ball A thrown ball travels in an arc Physics 100 Fall 2007 5 Questions about Falling Balls 1. Why does a dropped ball fall downward? 2. Do different balls fall at different rates? 3. Would a ball fall differently on the moon? 4. Can a ball move upward and still be falling? 5. Does a ball's horizontal motion affect its fall? Physics 100 Fall 2007 Question 1 Why does a dropped ball fall downward? What is gravity doing to the ball? Physics 100 Fall 2007 Gravity and Weight Gravity exerts a force on the ball That force is the ball's weight Since earth's gravity produces the ball's weight, that weight points toward the earth's center If let go, the ball's weight causes it to accelerate toward the earth's center (i.e. downward) Physics 100 Fall 2007 Newton's Law of Gravitation All bodies attract each other with a force described by: mM =m F=G r2 For us, M is the earth and if we are not in orbit, r is the earth's radius, and m g F F = mg F = m(GM/r 2) = mg. F = weight, m = mass Physics 100 Fall 2007 Earth M Question 2 Do different balls fall at different rates? - A. Yes; B. No. If different balls have different weights and different masses, is there any relationship between their accelerations as they fall? Physics 100 Fall 2007 Answer B Weight and Mass A ball's weight is proportional to its mass weight/mass = constant On earth's surface, weight/mass = 9.8 newtons/kilogram is the same for all balls (or other objects) is called "acceleration due to gravity" Physics 100 Fall 2007 Acceleration Due to Gravity Why this strange name? weight/mass force/mass = acceleration Acceleration due to gravity is an acceleration 9.8 newtons/kilogram = 9.8 meter/second2 On earth's surface, all falling balls accelerate downward at 9.8 meter/second2 Different balls fall at the same rate Physics 100 Fall 2007 Question 3 Would a ball fall differently on the moon? A. Yes; B. No. Yes! Moon's acceleration due to gravity is different. Physics 100 Fall 2007 Answer A Newton's Law of Gravitation All bodies attract each other with a force described by: mM =m F=G 2 r On the Moon, g is different because M and r are different than on Earth. m F F = mg. F = weight, m = mass Physics 100 Fall 2007 Moon M Question 4 Can a ball move upward and still be falling? How does falling affect a ball's i. acceleration? ii. velocity? iii. position? Physics 100 Fall 2007 A Falling Ball (Part 1) A falling ball accelerates downward steadily Its acceleration is constant and downward Its velocity increases in the downward direction When falling from rest (stationary), its velocity starts at zero and increases downward altitude decreases at an ever faster rate Physics 100 Fall 2007 Falling Downward Physics 100 Fall 2007 A Falling Ball (Part 2) A falling ball can start by heading upward. Its velocity starts in the upward direction Its inertia carries it up while gravity pulls down Its velocity becomes less and less upward Its altitude increases at an ever slower rate At some point, its velocity is momentarily zero Its velocity becomes more and more downward Its altitude decreases at ever faster rate Physics 100 Fall 2007 Falling Upward First Physics 100 Fall 2007 Quantitative Description of Motion Constant velocity (no acceleration) Position = starting position + velocity time x = x0 + vavg t Velocity = starting velocity v = v0 Constant acceleration (falling) Position: x = x0 + v0 t + () a t 2 Velocity = starting velocity + acceleration time v = v0 + a Average velocity: vavg = v0 + () a Physics 100 Fall 2007 t t Question 5 Does a ball's horizontal motion affect its fall? Why does a thrown ball travel in an arc? Physics 100 Fall 2007 Throws and Arcs I Gravity affects only the ball's vertical motion A ball coasts horizontally while falling vertically The two motions are independent Physics 100 Fall 2007 ...
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This note was uploaded on 03/26/2008 for the course PHYS 100 taught by Professor Tsui during the Fall '07 term at UNC.

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