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Unformatted text preview: Assignment Schedule
Monday Tuesday Wednesday 8/22 Pretest 1 posted Thursday Friday 8/24 HW 1 posted Monday 8/27 Pretest 1 due Pretest 2 posted Tuesday Wednesday 8/29 HW 1 due HW 2 posted Thursday Friday 8/31 Pretest 2 due Physics 100 Fall 2007 Observations about Skating When you're at rest on a level surface, without a push, you remain stationary. with a push, you start moving that direction. When you're moving on a level surface, without a push, you coast steady & straight. with a push, you change direction or speed. Physics 100 Fall 2007 Physical Quantities Position an object's location Velocity its change in position with time Vector Quantities Physics 100 Fall 2007 Motion at Constant Speed in a Line
Early time t1
1 Later time t2
2 Position x Position x Change in position
1 Velocity = Change in position Change in time x
2 x x x v= t
Physics 100 Fall 2007 Newton's First Law An object that is not subject to any outside forces moves at a constant velocity. At rest, the constant velocity is zero. This is a `natural' law. It describes physical phenomena which we observe.
Physics 100 Fall 2007 4 Questions about Skating
1. If a skater is at rest, why does she stay at rest? 2. If a skater is moving, why does she continue moving? 3. Why does a skater need ice or wheels to skate? 4. How does a skater start or stop moving?
Physics 100 Fall 2007 Question 3
Why does a skater need ice or wheels to skate? Why does a hovercraft need an air cushion? Why did I cut the banana instantly after letting it go? Physics 100 Fall 2007 Simplifying a Situation Real-world complications mask simple physics Solution: minimize or overwhelm complications To demonstrate inertia: work on level ground (goodbye gravity) use wheels, ice, or air support (goodbye friction) work fast (goodbye gravity, friction and air resistance)
Physics 100 Fall 2007 Physical Quantities and Units Position meter (m), foot (ft) inch, mile, lightyear, armlength Time second (s), second (s) minute, hour, day, year Velocity meter/second (m/s), foot/second (ft/s) miles/hour, furlong/fortnight
yellow = SI units purple = English units blue = Other units Physics 100 Fall 2007 Question 4
How does a skater start or stop moving?
i. What does a push do? ii. What does a skater respond to a push? iii. Do all skaters respond equally to equal pushes? Physics 100 Fall 2007 Physical Quantities Position an object's location Velocity change in position with time Force a push or a pull SI Units: newton (N), English Units: pound force (lbf) Acceleration change in velocity with time SI Units: (meter/second)/second = m/s2 English Units: (foot/second)/second = ft/s
Physics 100 Fall 2007 Changing Velocity in Straight Line
Car Speeding up
Early time Later time Car Slowing Down
Early time Later time v1 v1 v v2 v2 v1 v2 v1 v2 v a= t v v a= t Change in velocity Acceleration = Change in time speeding up velocity and acceleration: same direction slowing down velocity and acceleration: opposite direction BOTH are undergoing acceleration. Physics 100 Fall 2007 Changing Velocity on a Curve
Velocity (v) r v Acceleration (a)
2 1 r r v
a v1 v2 If you look where you are going, your velocity is straight ahead. Acceleration is towards the center of the curve. Physics 100 Fall 2007 Newton's Second Law
An object's acceleration is equal to the force exerted on it divided by its mass. That acceleration is in the same direction as the force. Acceleration = Force/Mass Force = Mass Acceleration F=m a
Physics 100 Fall 2007 Physical Quantities Position an object's location Velocity change in position with time Force a push or a pull Acceleration change in velocity with time Mass measure of object's inertia Units: kilogram (kg); pound mass (lbm) Physics 100 Fall 2007 Summary about Skating Skates can free you from external forces You normally coast constant velocity If you're at rest, you remain at rest If you're moving, you move steadily and straight When you experience external forces You accelerate you change velocity Acceleration depends on force and mass Physics 100 Fall 2007 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 ...
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- Fall '07